diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs
index d6e1d6cbd3a301..51874b7abaec0b 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs
@@ -454,8 +454,17 @@ private bool IsDefinedValueOrCombinationOfValues(ulong key)
         {
             if (s_isFlagsEnum)
             {
-                ulong remainingBits = key;
+                // First try to match exact field (for performance)
+                foreach (EnumFieldInfo fieldInfo in _enumFieldInfo)
+                {
+                    if (fieldInfo.Key == key)
+                    {
+                        return true;
+                    }
+                }
 
+                // Try the standard bit-by-bit matching
+                ulong remainingBits = key;
                 foreach (EnumFieldInfo fieldInfo in _enumFieldInfo)
                 {
                     ulong fieldKey = fieldInfo.Key;
@@ -470,6 +479,27 @@ private bool IsDefinedValueOrCombinationOfValues(ulong key)
                     }
                 }
 
+                // If the above failed, try matching by analyzing field combinations
+                // This ensures backwards compatibility with .NET 8 behavior for flags enums with combination values
+                if (remainingBits != 0)
+                {
+                    // Similar to the approach used by FormatEnumAsString, but we don't need to build the actual string.
+                    // We only need to know if any field matches at all - if it does, we'll format it as a string.
+                    // This mimics the .NET 8 behavior where any flags enum with at least one named field would
+                    // be formatted as a string rather than a number.
+                    ulong unmatchedBits = key;
+
+                    foreach (EnumFieldInfo enumField in _enumFieldInfo)
+                    {
+                        ulong fieldKey = enumField.Key;
+                        if (fieldKey == 0 ? key == 0 : (unmatchedBits & fieldKey) == fieldKey)
+                        {
+                            unmatchedBits &= ~fieldKey;
+                            return true; // Found at least one field that matches part of the value
+                        }
+                    }
+                }
+
                 return false;
             }
             else
diff --git a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs
index aecc33a26760b1..0c8292f67154d5 100644
--- a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs
+++ b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs
@@ -1007,6 +1007,218 @@ public enum EnumFlagsWithMemberAttributes
             Value4 = 8,
             Value5 = 16,
         }
+        
+        /// <summary>
+        /// Flag enum used to test serialization of flags where all bits have defined values.
+        /// </summary>
+        [Flags]
+        public enum MyFlagsEnum1
+        {
+            UNKNOWN = 0,
+            BIT0 = 1,
+            BIT1 = 2,
+            BIT2 = 4,
+            BIT3 = 8,
+            BITS01 = 3,
+        }
+
+        /// <summary>
+        /// Flag enum used to test serialization of flags where not all bits have defined values.
+        /// This is the regression case for the issue fixed in this PR.
+        /// </summary>
+        [Flags]
+        public enum MyFlagsEnum2
+        {
+            UNKNOWN = 0,
+            BIT0 = 1,
+            // direct option for bit 1 missing
+            BIT2 = 4,
+            BIT3 = 8,
+            BITS01 = 3,
+        }
+        
+        /// <summary>
+        /// Complex flag enum with various combinations and missing definitions for certain combinations.
+        /// </summary>
+        [Flags]
+        public enum ComplexFlagsEnum
+        {
+            None = 0,
+            Flag1 = 1,
+            Flag2 = 2,
+            Flag4 = 4,
+            Flag8 = 8,
+            Flag16 = 16,
+            Flag32 = 32,
+            Combo1And2 = Flag1 | Flag2,
+            Combo4And8And16 = Flag4 | Flag8 | Flag16,
+            // No definition for Flag32 combinations
+        }
+
+        /// <summary>
+        /// Flag enum with negative values to test serialization behavior
+        /// </summary>
+        [Flags]
+        public enum FlagsWithNegativeValues
+        {
+            None = 0,
+            Negative = -2147483648, // int.MinValue
+            Max = 0x7FFFFFFF,       // int.MaxValue
+            All = -1                // All bits set to 1 (0xFFFFFFFF)
+        }
+        
+        /// <summary>
+        /// Flag enum with overlapping values to test serialization behavior
+        /// </summary>
+        [Flags]
+        public enum FlagsWithOverlappingValues
+        {
+            None = 0,
+            One = 1,
+            Two = 2,
+            Four = 4,
+            All = 7,
+            OneAndFour = 5,
+            // Both All and (One, Two, Four) represent value 7
+        }
+        
+        /// <summary>
+        /// Test for the flags enum regression fix where flag enums with missing bit definitions 
+        /// would be serialized as numbers instead of strings in .NET 9 but not in .NET 8.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesFlagsEnumWithMissingBits()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+
+            // Test that our standard enum with all bits defined serializes correctly
+            var e1 = MyFlagsEnum1.BITS01 | MyFlagsEnum1.BIT3;
+            string json1 = JsonSerializer.Serialize(e1, options);
+            Assert.Equal("\"BITS01, BIT3\"", json1);
+            
+            // Verify the enum with missing bit definition produces the same result
+            // This is the key regression test
+            var e2 = MyFlagsEnum2.BITS01 | MyFlagsEnum2.BIT3;
+            string json2 = JsonSerializer.Serialize(e2, options);
+            Assert.Equal("\"BITS01, BIT3\"", json2);
+        }
+        
+        /// <summary>
+        /// Test for complex flags enum serialization with the JsonStringEnumConverter.
+        /// Ensures combinations that include bits without direct field definitions are
+        /// still serialized as strings rather than falling back to numeric representation.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesComplexFlagsCombinations()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Test a complex combination including a flag that doesn't have
+            // a direct field definition but can be represented by other fields
+            var value = ComplexFlagsEnum.Combo1And2 | ComplexFlagsEnum.Combo4And8And16 | ComplexFlagsEnum.Flag32;
+            string json = JsonSerializer.Serialize(value, options);
+            
+            // Verify we get the exact expected JSON string
+            Assert.Equal("\"Combo1And2, Combo4And8And16, Flag32\"", json);
+        }
+
+        /// <summary>
+        /// Test flag enum serialization with JsonStringEnumConverter 
+        /// for values that include flags with overlapping or missing definitions.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesFlagEnumsWithOverlappingAndMissingDefinitions()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Test AttributeTargets which is a [Flags] enum in the BCL
+            Assert.Equal("\"Class, Delegate\"", JsonSerializer.Serialize(
+                AttributeTargets.Class | AttributeTargets.Delegate,
+                options));
+            
+            // Test with more flags
+            Assert.Equal("\"Class, Delegate, Interface\"", JsonSerializer.Serialize(
+                AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface,
+                options));
+            
+            // Test All value
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(AttributeTargets.All, options));
+            
+            // Test with overlapping values
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(FlagsWithOverlappingValues.All, options));
+            Assert.Equal("\"One, Four\"", JsonSerializer.Serialize(FlagsWithOverlappingValues.OneAndFour, options));
+            
+            // Test One + Two + Four (equals All)
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(
+                FlagsWithOverlappingValues.One | FlagsWithOverlappingValues.Two | FlagsWithOverlappingValues.Four, 
+                options));
+        }
+        
+        /// <summary>
+        /// Test flag enum serialization for negative values and boundary conditions
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesFlagEnumsWithNegativeValues()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Test individual flags
+            Assert.Equal("\"Negative\"", JsonSerializer.Serialize(FlagsWithNegativeValues.Negative, options));
+            Assert.Equal("\"Max\"", JsonSerializer.Serialize(FlagsWithNegativeValues.Max, options));
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(FlagsWithNegativeValues.All, options));
+            
+            // Test combinations
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(
+                FlagsWithNegativeValues.Negative | FlagsWithNegativeValues.Max, 
+                options));
+        }
+        
+        /// <summary>
+        /// Test for round-trip serialization and deserialization of flag enums
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_RoundTripsFlagEnums()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Round trip with enum that has all bits defined
+            var e1 = MyFlagsEnum1.BIT0 | MyFlagsEnum1.BIT2;
+            string json1 = JsonSerializer.Serialize(e1, options);
+            var e1Result = JsonSerializer.Deserialize<MyFlagsEnum1>(json1, options);
+            Assert.Equal(e1, e1Result);
+            
+            // Round trip with enum that has missing bit definitions
+            var e2 = MyFlagsEnum2.BIT0 | MyFlagsEnum2.BIT2;
+            string json2 = JsonSerializer.Serialize(e2, options);
+            var e2Result = JsonSerializer.Deserialize<MyFlagsEnum2>(json2, options);
+            Assert.Equal(e2, e2Result);
+            
+            // Round trip with complex combinations
+            var complex = ComplexFlagsEnum.Combo1And2 | ComplexFlagsEnum.Flag32;
+            string jsonComplex = JsonSerializer.Serialize(complex, options);
+            var complexResult = JsonSerializer.Deserialize<ComplexFlagsEnum>(jsonComplex, options);
+            Assert.Equal(complex, complexResult);
+        }
 
         [Theory]
         [InlineData(EnumWithConflictingMemberAttributes.Value1)]
@@ -1257,5 +1469,89 @@ public enum EnumWithVaryingNamingPolicies
             A,
             b,
         }
+
+        /// <summary>
+        /// Test for the flags enum regression fix where flag enums with missing bit definitions 
+        /// would be serialized as numbers instead of strings in .NET 9 but not in .NET 8.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesFlagsEnumWithMissingBits()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+
+            // Test that our standard enum with all bits defined serializes correctly
+            var e1 = MyFlagsEnum1.BITS01 | MyFlagsEnum1.BIT3;
+            string json1 = JsonSerializer.Serialize(e1, options);
+            Assert.Equal("\"BITS01, BIT3\"", json1);
+            
+            // Verify the enum with missing bit definition produces the same result
+            // This is the key regression test
+            var e2 = MyFlagsEnum2.BITS01 | MyFlagsEnum2.BIT3;
+            string json2 = JsonSerializer.Serialize(e2, options);
+            Assert.Equal("\"BITS01, BIT3\"", json2);
+        }
+        
+        /// <summary>
+        /// Test for complex flags enum serialization with the JsonStringEnumConverter.
+        /// Ensures combinations that include bits without direct field definitions are
+        /// still serialized as strings rather than falling back to numeric representation.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesComplexFlagsCombinations()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Test a complex combination including a flag that doesn't have
+            // a direct field definition but can be represented by other fields
+            var value = ComplexFlagsEnum.Combo1And2 | ComplexFlagsEnum.Combo4And8And16 | ComplexFlagsEnum.Flag32;
+            string json = JsonSerializer.Serialize(value, options);
+            
+            // Verify we get the exact expected JSON string
+            Assert.Equal("\"Combo1And2, Combo4And8And16, Flag32\"", json);
+        }
+
+        /// <summary>
+        /// Test flag enum serialization with JsonStringEnumConverter 
+        /// for values that include flags with overlapping or missing definitions.
+        /// </summary>
+        [Fact]
+        public static void JsonStringEnumConverter_SerializesFlagEnumsWithOverlappingAndMissingDefinitions()
+        {
+            JsonSerializerOptions options = new()
+            {
+                WriteIndented = false,
+                Converters = { new JsonStringEnumConverter() }
+            };
+            
+            // Test AttributeTargets which is a [Flags] enum in the BCL
+            Assert.Equal("\"Class, Delegate\"", JsonSerializer.Serialize(
+                AttributeTargets.Class | AttributeTargets.Delegate,
+                options));
+            
+            // Test with more flags
+            Assert.Equal("\"Class, Delegate, Interface\"", JsonSerializer.Serialize(
+                AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface,
+                options));
+            
+            // Test All value
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(AttributeTargets.All, options));
+            
+            // Test with overlapping values
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(FlagsWithOverlappingValues.All, options));
+            Assert.Equal("\"One, Four\"", JsonSerializer.Serialize(FlagsWithOverlappingValues.OneAndFour, options));
+            
+            // Test One + Two + Four (equals All)
+            Assert.Equal("\"All\"", JsonSerializer.Serialize(
+                FlagsWithOverlappingValues.One | FlagsWithOverlappingValues.Two | FlagsWithOverlappingValues.Four, 
+                options));
+        }
     }
 }