diff --git a/lang/funcs/collection.go b/lang/funcs/collection.go
index ee3ad2b3a..5422a0f9d 100644
--- a/lang/funcs/collection.go
+++ b/lang/funcs/collection.go
@@ -427,32 +427,47 @@ var FlattenFunc = function.New(&function.Spec{
 	Params: []function.Parameter{
 		{
 			Name: "list",
-			Type: cty.List(cty.DynamicPseudoType),
+			Type: cty.DynamicPseudoType,
 		},
 	},
-	Type: function.StaticReturnType(cty.List(cty.DynamicPseudoType)),
+	Type: func(args []cty.Value) (cty.Type, error) {
+		if !args[0].IsWhollyKnown() {
+			return cty.DynamicPseudoType, nil
+		}
+
+		argTy := args[0].Type()
+		if !argTy.IsListType() && !argTy.IsSetType() && !argTy.IsTupleType() {
+			return cty.NilType, fmt.Errorf("can only flatten lists, sets and tuples")
+		}
+
+		outputList := make([]cty.Value, 0)
+		retVal := flattener(outputList, args[0])
+		tys := make([]cty.Type, len(retVal))
+		for i, ty := range retVal {
+			tys[i] = ty.Type()
+		}
+		return cty.Tuple(tys), nil
+	},
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
 		inputList := args[0]
 		if !inputList.IsWhollyKnown() {
 			return cty.UnknownVal(retType), nil
 		}
-
 		if inputList.LengthInt() == 0 {
-			return cty.ListValEmpty(retType.ElementType()), nil
+			return cty.EmptyTupleVal, nil
 		}
 		outputList := make([]cty.Value, 0)
 
-		return cty.ListVal(flattener(outputList, inputList)), nil
+		return cty.TupleVal(flattener(outputList, inputList)), nil
 	},
 })
 
 // Flatten until it's not a cty.List
 func flattener(finalList []cty.Value, flattenList cty.Value) []cty.Value {
-
 	for it := flattenList.ElementIterator(); it.Next(); {
 		_, val := it.Element()
 
-		if val.Type().IsListType() {
+		if val.Type().IsListType() || val.Type().IsSetType() || val.Type().IsTupleType() {
 			finalList = flattener(finalList, val)
 		} else {
 			finalList = append(finalList, val)
diff --git a/lang/funcs/collection_test.go b/lang/funcs/collection_test.go
index ac686fa72..4339d3af4 100644
--- a/lang/funcs/collection_test.go
+++ b/lang/funcs/collection_test.go
@@ -1035,7 +1035,7 @@ func TestFlatten(t *testing.T) {
 					cty.StringVal("d"),
 				}),
 			}),
-			cty.ListVal([]cty.Value{
+			cty.TupleVal([]cty.Value{
 				cty.StringVal("a"),
 				cty.StringVal("b"),
 				cty.StringVal("c"),
@@ -1054,12 +1054,50 @@ func TestFlatten(t *testing.T) {
 					cty.StringVal("d"),
 				}),
 			}),
-			cty.UnknownVal(cty.List(cty.DynamicPseudoType)),
+			cty.DynamicVal,
 			false,
 		},
 		{
 			cty.ListValEmpty(cty.String),
-			cty.ListValEmpty(cty.DynamicPseudoType),
+			cty.EmptyTupleVal,
+			false,
+		},
+		{
+			cty.SetVal([]cty.Value{
+				cty.SetVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.SetVal([]cty.Value{
+					cty.StringVal("c"),
+					cty.StringVal("d"),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+				cty.StringVal("d"),
+			}),
+			false,
+		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.SetVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.ListVal([]cty.Value{
+					cty.StringVal("c"),
+					cty.StringVal("d"),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+				cty.StringVal("d"),
+			}),
 			false,
 		},
 	}
diff --git a/lang/functions_test.go b/lang/functions_test.go
index 9bab6887d..9233d0ff9 100644
--- a/lang/functions_test.go
+++ b/lang/functions_test.go
@@ -309,8 +309,8 @@ func TestFunctions(t *testing.T) {
 
 		"flatten": {
 			{
-				`flatten([tolist(["a", "b"]), tolist(["c", "d"])])`,
-				cty.ListVal([]cty.Value{
+				`flatten([["a", "b"], ["c", "d"]])`,
+				cty.TupleVal([]cty.Value{
 					cty.StringVal("a"),
 					cty.StringVal("b"),
 					cty.StringVal("c"),