diff --git a/src/idl_gen_ts.cpp b/src/idl_gen_ts.cpp
index 7bee3f80148..acd2a4febec 100644
--- a/src/idl_gen_ts.cpp
+++ b/src/idl_gen_ts.cpp
@@ -408,7 +408,7 @@ class TsGenerator : public BaseGenerator {
     switch (type.base_type) {
       case BASE_TYPE_BOOL:
       case BASE_TYPE_CHAR: return "Int8";
-      case BASE_TYPE_UTYPE:
+      case BASE_TYPE_UTYPE: return GenType(GetUnionUnderlyingType(type));
       case BASE_TYPE_UCHAR: return "Uint8";
       case BASE_TYPE_SHORT: return "Int16";
       case BASE_TYPE_USHORT: return "Uint16";
@@ -562,11 +562,26 @@ class TsGenerator : public BaseGenerator {
     }
   }
 
+  static Type GetUnionUnderlyingType(const Type &type)
+  {
+    if (type.enum_def != nullptr && 
+        type.enum_def->underlying_type.base_type != type.base_type) {
+      return type.enum_def->underlying_type;
+    } else {
+        return Type(BASE_TYPE_UCHAR);
+    }
+  }
+
+  static Type GetUnderlyingVectorType(const Type &vector_type)
+  {
+    return (vector_type.base_type == BASE_TYPE_UTYPE) ? GetUnionUnderlyingType(vector_type) : vector_type;
+  }
+
   // Returns the method name for use with add/put calls.
   std::string GenWriteMethod(const Type &type) {
     // Forward to signed versions since unsigned versions don't exist
     switch (type.base_type) {
-      case BASE_TYPE_UTYPE:
+      case BASE_TYPE_UTYPE: return GenWriteMethod(GetUnionUnderlyingType(type));
       case BASE_TYPE_UCHAR: return GenWriteMethod(Type(BASE_TYPE_CHAR));
       case BASE_TYPE_USHORT: return GenWriteMethod(Type(BASE_TYPE_SHORT));
       case BASE_TYPE_UINT: return GenWriteMethod(Type(BASE_TYPE_INT));
@@ -1763,7 +1778,8 @@ class TsGenerator : public BaseGenerator {
             auto vectortype = field.value.type.VectorType();
             auto vectortypename =
                 GenTypeName(imports, struct_def, vectortype, false);
-            auto inline_size = InlineSize(vectortype);
+            auto type = GetUnderlyingVectorType(vectortype);
+            auto inline_size = InlineSize(type);
             auto index = GenBBAccess() +
                          ".__vector(this.bb_pos + offset) + index" +
                          MaybeScale(inline_size);
@@ -1994,8 +2010,9 @@ class TsGenerator : public BaseGenerator {
 
         if (IsVector(field.value.type)) {
           auto vector_type = field.value.type.VectorType();
-          auto alignment = InlineAlignment(vector_type);
-          auto elem_size = InlineSize(vector_type);
+          auto type = GetUnderlyingVectorType(vector_type);
+          auto alignment = InlineAlignment(type);
+          auto elem_size = InlineSize(type);
 
           // Generate a method to create a vector from a JavaScript array
           if (!IsStruct(vector_type)) {
diff --git a/src/idl_parser.cpp b/src/idl_parser.cpp
index 4df4558ce95..2b401cf0c0b 100644
--- a/src/idl_parser.cpp
+++ b/src/idl_parser.cpp
@@ -2718,7 +2718,7 @@ bool Parser::Supports64BitOffsets() const {
 }
 
 bool Parser::SupportsUnionUnderlyingType() const {
-    return (opts.lang_to_generate & ~IDLOptions::kCpp) == 0;
+    return (opts.lang_to_generate & ~(IDLOptions::kCpp | IDLOptions::kTs)) == 0;
 }
 
 Namespace *Parser::UniqueNamespace(Namespace *ns) {
diff --git a/tests/parser_test.cpp b/tests/parser_test.cpp
index d63c0b180e4..33b1d6e1e6d 100644
--- a/tests/parser_test.cpp
+++ b/tests/parser_test.cpp
@@ -845,7 +845,7 @@ void ParseUnionTest() {
   // Test union underlying type
   const char *source = "table A {} table B {} union U : int {A, B} table C {test_union: U; test_vector_of_union: [U];}";
   flatbuffers::Parser parser3;
-  parser3.opts.lang_to_generate = flatbuffers::IDLOptions::kCpp;
+  parser3.opts.lang_to_generate = flatbuffers::IDLOptions::kCpp | flatbuffers::IDLOptions::kTs;
   TEST_EQ(parser3.Parse(source), true);
   
   parser3.opts.lang_to_generate &= flatbuffers::IDLOptions::kJava;
diff --git a/tests/ts/JavaScriptUnionUnderlyingTypeTest.js b/tests/ts/JavaScriptUnionUnderlyingTypeTest.js
new file mode 100644
index 00000000000..6a324ca43ec
--- /dev/null
+++ b/tests/ts/JavaScriptUnionUnderlyingTypeTest.js
@@ -0,0 +1,26 @@
+import assert from 'assert'
+import * as flatbuffers from 'flatbuffers'
+import {UnionUnderlyingType as Test} from './union_underlying_type_test.js'
+
+function main() {
+    let a = new Test.AT();
+    a.a = 1;
+    let b = new Test.BT();
+    b.b = "foo";
+    let c = new Test.CT();
+    c.c = true;
+    let d = new Test.DT();
+    d.testUnionType = Test.ABC.A;
+    d.testUnion = a;
+    d.testVectorOfUnionType = [Test.ABC.A, Test.ABC.B, Test.ABC.C];
+    d.testVectorOfUnion = [a, b, c];
+
+    let fbb = new flatbuffers.Builder();
+    let offset = d.pack(fbb);
+    fbb.finish(offset);
+
+    let unpacked = Test.D.getRootAsD(fbb.dataBuffer()).unpack();
+    assert.equal(JSON.stringify(unpacked), JSON.stringify(d));
+}
+
+main()
\ No newline at end of file
diff --git a/tests/ts/TypeScriptTest.py b/tests/ts/TypeScriptTest.py
index ae357ef09c4..c6c7cb7f993 100755
--- a/tests/ts/TypeScriptTest.py
+++ b/tests/ts/TypeScriptTest.py
@@ -117,6 +117,11 @@ def esbuild(input, output):
 )
 esbuild("typescript_keywords.ts", "typescript_keywords_generated.cjs")
 
+flatc(
+    options=["--ts", "--reflect-names", "--gen-name-strings", "--gen-mutable", "--gen-object-api", "--ts-entry-points", "--ts-flat-files"],
+    schema="../union_underlying_type_test.fbs"
+)
+
 print("Running TypeScript Compiler...")
 check_call(["tsc"])
 print("Running TypeScript Compiler in old node resolution mode for no_import_ext...")
@@ -129,6 +134,7 @@ def esbuild(input, output):
 check_call(NODE_CMD + ["JavaScriptUnionVectorTest"])
 check_call(NODE_CMD + ["JavaScriptFlexBuffersTest"])
 check_call(NODE_CMD + ["JavaScriptComplexArraysTest"])
+check_call(NODE_CMD + ["JavaScriptUnionUnderlyingTypeTest"])
 
 print("Running old v1 TypeScript Tests...")
 check_call(NODE_CMD + ["JavaScriptTestv1.cjs", "./monster_test_generated.cjs"])
diff --git a/tests/ts/tsconfig.json b/tests/ts/tsconfig.json
index d9ef7410c3a..eb08992b427 100644
--- a/tests/ts/tsconfig.json
+++ b/tests/ts/tsconfig.json
@@ -14,6 +14,7 @@
     "optional_scalars/**/*.ts",
     "namespace_test/**/*.ts",
     "union_vector/**/*.ts",
-    "arrays_test_complex/**/*.ts"
+    "arrays_test_complex/**/*.ts",
+    "union_underlying_type_test.ts"
   ]
 }