diff --git a/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs b/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs
index 65cc4012fb82b..b2b2641a2fd89 100644
--- a/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs
+++ b/src/Compilers/CSharp/Portable/Parser/LanguageParser.cs
@@ -4880,8 +4880,8 @@ private void ParseParameterModifiers(SyntaxListBuilder modifiers, bool isFunctio
                 {
                     modifiers.Add(scopedKeyword);
 
-                    // Look if ref/out/in/readonly are next
-                    while (this.CurrentToken.Kind is SyntaxKind.RefKeyword or SyntaxKind.OutKeyword or SyntaxKind.InKeyword or SyntaxKind.ReadOnlyKeyword)
+                    // Look if ref/out/in/readonly/this are next
+                    while (this.CurrentToken.Kind is SyntaxKind.RefKeyword or SyntaxKind.OutKeyword or SyntaxKind.InKeyword or SyntaxKind.ReadOnlyKeyword or SyntaxKind.ThisKeyword)
                     {
                         modifiers.Add(this.EatToken());
                     }
diff --git a/src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_LifetimeAnnotation.cs b/src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_LifetimeAnnotation.cs
index 38c30ad1dcda4..457880e546c26 100644
--- a/src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_LifetimeAnnotation.cs
+++ b/src/Compilers/CSharp/Test/Emit3/Attributes/AttributeTests_LifetimeAnnotation.cs
@@ -560,6 +560,66 @@ [ScopedRef] scoped R r
                 });
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void EmitAttribute_ExtensionMethodParameters_ScopedInThis()
+        {
+            var source =
+@"static class Extensions
+{
+    public static void F(scoped in this int i) { }
+}";
+            var comp = CreateCompilation(source);
+            var expected =
+@"void Extensions.F(this scoped in System.Int32 i)
+    [ScopedRef] scoped in System.Int32 i
+";
+            CompileAndVerify(comp, symbolValidator: module =>
+            {
+                Assert.Equal("System.Runtime.CompilerServices.ScopedRefAttribute", GetScopedRefType(module).ToTestDisplayString());
+                AssertScopedRefAttributes(module, expected);
+            });
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void EmitAttribute_ExtensionMethodParameters_ScopedRefThis()
+        {
+            var source =
+@"static class Extensions
+{
+    public static void F(scoped ref this int i) { }
+}";
+            var comp = CreateCompilation(source);
+            var expected =
+@"void Extensions.F(this scoped ref System.Int32 i)
+    [ScopedRef] scoped ref System.Int32 i
+";
+            CompileAndVerify(comp, symbolValidator: module =>
+            {
+                Assert.Equal("System.Runtime.CompilerServices.ScopedRefAttribute", GetScopedRefType(module).ToTestDisplayString());
+                AssertScopedRefAttributes(module, expected);
+            });
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void EmitAttribute_ExtensionMethodParameters_ScopedRefReadonlyThis()
+        {
+            var source =
+@"static class Extensions
+{
+    public static void F(scoped ref readonly this int i) { }
+}";
+            var comp = CreateCompilation(source);
+            var expected =
+@"void Extensions.F(this scoped ref readonly System.Int32 i)
+    [ScopedRef] scoped ref readonly System.Int32 i
+";
+            CompileAndVerify(comp, symbolValidator: module =>
+            {
+                Assert.Equal("System.Runtime.CompilerServices.ScopedRefAttribute", GetScopedRefType(module).ToTestDisplayString());
+                AssertScopedRefAttributes(module, expected);
+            });
+        }
+
         private static void AssertScopedRefAttributes(ModuleSymbol module, string expected)
         {
             var actual = ScopedRefAttributesVisitor.GetString((PEModuleSymbol)module);
diff --git a/src/Compilers/CSharp/Test/Syntax/Parsing/DeclarationScopeParsingTests.cs b/src/Compilers/CSharp/Test/Syntax/Parsing/DeclarationScopeParsingTests.cs
index 6740c40c0c393..82b1632859b52 100644
--- a/src/Compilers/CSharp/Test/Syntax/Parsing/DeclarationScopeParsingTests.cs
+++ b/src/Compilers/CSharp/Test/Syntax/Parsing/DeclarationScopeParsingTests.cs
@@ -14944,5 +14944,144 @@ public void Catch_03()
             }
             EOF();
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void ScopedWithThis_01()
+        {
+            var source = "void M(scoped in this int p);";
+            UsingDeclaration(source);
+
+            N(SyntaxKind.MethodDeclaration);
+            {
+                N(SyntaxKind.PredefinedType);
+                {
+                    N(SyntaxKind.VoidKeyword);
+                }
+                N(SyntaxKind.IdentifierToken, "M");
+                N(SyntaxKind.ParameterList);
+                {
+                    N(SyntaxKind.OpenParenToken);
+                    N(SyntaxKind.Parameter);
+                    {
+                        N(SyntaxKind.ScopedKeyword);
+                        N(SyntaxKind.InKeyword);
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.PredefinedType);
+                        {
+                            N(SyntaxKind.IntKeyword);
+                        }
+                        N(SyntaxKind.IdentifierToken, "p");
+                    }
+                    N(SyntaxKind.CloseParenToken);
+                }
+                N(SyntaxKind.SemicolonToken);
+            }
+            EOF();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void ScopedWithThis_02()
+        {
+            var source = "void M(scoped ref this int p);";
+            UsingDeclaration(source);
+
+            N(SyntaxKind.MethodDeclaration);
+            {
+                N(SyntaxKind.PredefinedType);
+                {
+                    N(SyntaxKind.VoidKeyword);
+                }
+                N(SyntaxKind.IdentifierToken, "M");
+                N(SyntaxKind.ParameterList);
+                {
+                    N(SyntaxKind.OpenParenToken);
+                    N(SyntaxKind.Parameter);
+                    {
+                        N(SyntaxKind.ScopedKeyword);
+                        N(SyntaxKind.RefKeyword);
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.PredefinedType);
+                        {
+                            N(SyntaxKind.IntKeyword);
+                        }
+                        N(SyntaxKind.IdentifierToken, "p");
+                    }
+                    N(SyntaxKind.CloseParenToken);
+                }
+                N(SyntaxKind.SemicolonToken);
+            }
+            EOF();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void ScopedWithThis_03()
+        {
+            // Test scoped ref readonly this
+            var source = "void M(scoped ref readonly this int p);";
+            UsingDeclaration(source);
+
+            N(SyntaxKind.MethodDeclaration);
+            {
+                N(SyntaxKind.PredefinedType);
+                {
+                    N(SyntaxKind.VoidKeyword);
+                }
+                N(SyntaxKind.IdentifierToken, "M");
+                N(SyntaxKind.ParameterList);
+                {
+                    N(SyntaxKind.OpenParenToken);
+                    N(SyntaxKind.Parameter);
+                    {
+                        N(SyntaxKind.ScopedKeyword);
+                        N(SyntaxKind.RefKeyword);
+                        N(SyntaxKind.ReadOnlyKeyword);
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.PredefinedType);
+                        {
+                            N(SyntaxKind.IntKeyword);
+                        }
+                        N(SyntaxKind.IdentifierToken, "p");
+                    }
+                    N(SyntaxKind.CloseParenToken);
+                }
+                N(SyntaxKind.SemicolonToken);
+            }
+            EOF();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78491")]
+        public void ScopedWithThis_04()
+        {
+            // Test that this scoped in still works
+            var source = "void M(this scoped in int p);";
+            UsingDeclaration(source);
+
+            N(SyntaxKind.MethodDeclaration);
+            {
+                N(SyntaxKind.PredefinedType);
+                {
+                    N(SyntaxKind.VoidKeyword);
+                }
+                N(SyntaxKind.IdentifierToken, "M");
+                N(SyntaxKind.ParameterList);
+                {
+                    N(SyntaxKind.OpenParenToken);
+                    N(SyntaxKind.Parameter);
+                    {
+                        N(SyntaxKind.ThisKeyword);
+                        N(SyntaxKind.ScopedKeyword);
+                        N(SyntaxKind.InKeyword);
+                        N(SyntaxKind.PredefinedType);
+                        {
+                            N(SyntaxKind.IntKeyword);
+                        }
+                        N(SyntaxKind.IdentifierToken, "p");
+                    }
+                    N(SyntaxKind.CloseParenToken);
+                }
+                N(SyntaxKind.SemicolonToken);
+            }
+            EOF();
+        }
     }
 }