diff --git a/DESCRIPTION b/DESCRIPTION
index 6afa9987..b28ecb0b 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -50,6 +50,7 @@ Imports:
     cli,
     dplyr,
     ggrepel,
+    gmp,
     magrittr,
     purrr,
     r2dii.analysis (>= 0.5.1),
diff --git a/NEWS.md b/NEWS.md
index 266f8ff5..78b1cffe 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,6 +1,7 @@
 # pacta.loanbook (development version)
 
 * fixed bug in `pacta_loanbook_deps()` (#229)
+* added `is_valid_lei()` (#150)
 
 # pacta.loanbook 0.1.0
 
diff --git a/R/is_valid_lei.R b/R/is_valid_lei.R
new file mode 100644
index 00000000..0503a17d
--- /dev/null
+++ b/R/is_valid_lei.R
@@ -0,0 +1,33 @@
+is_valid_lei <- function(x) {
+  if (is.data.frame(x) && identical(length(x), 1L)) {
+    x <- x[[1L]]
+  }
+
+  x[is.na(x)] <- "XXX" # set NAs to something sure to return FALSE
+  x_factored <- as.factor(x)
+  x_levels <- levels(x_factored)
+
+  x_levels <- toupper(x_levels)
+  x_levels <- gsub(pattern = "[[:blank:]]", replacement = "", x_levels)
+  valid_struct <- grepl(pattern = "^[[:alnum:][:digit:]]{20}$", x = x_levels)
+
+  valid_lei <-
+    vapply(
+      X = x_levels,
+      FUN = function(x) {
+        x <-
+          pmatch(
+            x = unlist(strsplit(x, split = NULL, fixed = TRUE)),
+            table = c(0:9, LETTERS),
+            duplicates.ok = TRUE
+          )
+        x <- paste(x - 1L, collapse = "")
+        x <- sub(pattern = "^0+", replacement = "", x = x)
+        gmp::mod.bigz(x, 97) == 1
+      },
+      FUN.VALUE = logical(1),
+      USE.NAMES = FALSE
+    )
+
+  valid_struct[as.numeric(x_factored)] & valid_lei[as.numeric(x_factored)]
+}
diff --git a/tests/testthat/test-is_valid_lei.R b/tests/testthat/test-is_valid_lei.R
new file mode 100644
index 00000000..6ce50e37
--- /dev/null
+++ b/tests/testthat/test-is_valid_lei.R
@@ -0,0 +1,76 @@
+test_that("correctly identifies valid LEIs", {
+  expect_true(is_valid_lei("11111111111111111104")) # manufactured valid
+  expect_true(is_valid_lei("XXXXXXXXXXXXXXXXXX35")) # manufactured valid
+  expect_true(is_valid_lei("xxxxxxxxxxxxxxxxxx35")) # manufactured valid with lowercase
+  expect_true(is_valid_lei("54930084UKLVMY22DS16")) # known valid
+  expect_true(is_valid_lei("213800WSGIIZCXF1P572")) # known valid
+  expect_true(is_valid_lei("5493000IBP32UQZ0KL24")) # known valid
+  expect_true(is_valid_lei("L3I9ZG2KFGXZ61BMYR72")) # known valid
+  expect_true(is_valid_lei(" L3I9ZG2KFGXZ61BMYR72 ")) # with leading/trailing space
+  expect_true(is_valid_lei("L3I9 ZG2KFGXZ61BMYR 72")) # with internal space
+})
+
+test_that("correctly identifies invalid LEIs", {
+  expect_false(is_valid_lei("18500033")) # too short
+  expect_false(is_valid_lei("?8500033XH6RG332SX89")) # non-alphanumeric character
+  expect_false(is_valid_lei("XXXXXXXXXXXXXXXXXX00")) # manufactured invalid
+})
+
+test_that("validates multiple LEIs", {
+  leis <- c(invalid_lei = "XXX", valid_lei = "XXXXXXXXXXXXXXXXXX35")
+  expect_identical(is_valid_lei(leis), c(FALSE, TRUE))
+  expect_identical(is_valid_lei(data.frame(leis)[1L]), c(FALSE, TRUE))
+})
+
+test_that("always outputs a logical vector", {
+  # invalid and valid codes
+  leis <- c(invalid_lei = "XXX", valid_lei = "XXXXXXXXXXXXXXXXXX35")
+  expect_vector(is_valid_lei(leis[1L]), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(leis[2L]), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(leis), ptype = logical(), size = 2L)
+
+  # expected possible uses
+  leis_df <- data.frame(lei = leis)
+
+  out <- dplyr::mutate(leis_df, valid_lei = is_valid_lei(lei))$valid_lei
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(leis_df$lei)
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(leis_df["lei"])
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(leis_df[1L])
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(leis_df[["lei"]])
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(leis_df[[1L]])
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  out <- is_valid_lei(tibble::as_tibble(leis_df)["lei"])
+  expect_vector(out, ptype = logical(), size = 2L)
+
+  # unexpected input types
+  expect_vector(is_valid_lei(1L), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(1L:2L), ptype = logical(), size = 2L)
+  expect_vector(is_valid_lei(TRUE), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(FALSE), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(c(TRUE, FALSE)), ptype = logical(), size = 2L)
+  expect_vector(is_valid_lei(NA), ptype = logical(), size = 1L)
+  expect_vector(is_valid_lei(c(NA, NA)), ptype = logical(), size = 2L)
+})
+
+test_that("returns `FALSE` for unexpected values", {
+  expect_identical(is_valid_lei(NA), FALSE)
+  expect_identical(is_valid_lei(NaN), FALSE)
+  expect_identical(is_valid_lei(Inf), FALSE)
+  expect_identical(is_valid_lei(NA_integer_), FALSE)
+  expect_identical(is_valid_lei(NA_real_), FALSE)
+  expect_identical(is_valid_lei(NA_complex_), FALSE)
+  expect_identical(is_valid_lei(NA_character_), FALSE)
+  expect_identical(is_valid_lei(c(TRUE, FALSE)), c(FALSE, FALSE))
+  expect_identical(is_valid_lei(1L:2L), c(FALSE, FALSE))
+})