diff --git a/lang/funcs/collection.go b/lang/funcs/collection.go
index ce6c8dc8f..0536dcfdf 100644
--- a/lang/funcs/collection.go
+++ b/lang/funcs/collection.go
@@ -20,6 +20,7 @@ var LengthFunc = function.New(&function.Spec{
 			Type:             cty.DynamicPseudoType,
 			AllowDynamicType: true,
 			AllowUnknown:     true,
+			AllowMarked:      true,
 		},
 	},
 	Type: func(args []cty.Value) (cty.Type, error) {
@@ -34,15 +35,16 @@ var LengthFunc = function.New(&function.Spec{
 	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
 		coll := args[0]
 		collTy := args[0].Type()
+		marks := coll.Marks()
 		switch {
 		case collTy == cty.DynamicPseudoType:
-			return cty.UnknownVal(cty.Number), nil
+			return cty.UnknownVal(cty.Number).WithMarks(marks), nil
 		case collTy.IsTupleType():
 			l := len(collTy.TupleElementTypes())
-			return cty.NumberIntVal(int64(l)), nil
+			return cty.NumberIntVal(int64(l)).WithMarks(marks), nil
 		case collTy.IsObjectType():
 			l := len(collTy.AttributeTypes())
-			return cty.NumberIntVal(int64(l)), nil
+			return cty.NumberIntVal(int64(l)).WithMarks(marks), nil
 		case collTy == cty.String:
 			// We'll delegate to the cty stdlib strlen function here, because
 			// it deals with all of the complexities of tokenizing unicode
diff --git a/lang/funcs/collection_test.go b/lang/funcs/collection_test.go
index 9cc428a91..974a8437d 100644
--- a/lang/funcs/collection_test.go
+++ b/lang/funcs/collection_test.go
@@ -122,6 +122,54 @@ func TestLength(t *testing.T) {
 			cty.DynamicVal,
 			cty.UnknownVal(cty.Number),
 		},
+		{ // Marked collections return a marked length
+			cty.ListVal([]cty.Value{
+				cty.StringVal("hello"),
+				cty.StringVal("world"),
+			}).Mark("secret"),
+			cty.NumberIntVal(2).Mark("secret"),
+		},
+		{ // Marks on values in unmarked collections do not propagate
+			cty.ListVal([]cty.Value{
+				cty.StringVal("hello").Mark("a"),
+				cty.StringVal("world").Mark("b"),
+			}),
+			cty.NumberIntVal(2),
+		},
+		{ // Marked strings return a marked length
+			cty.StringVal("hello world").Mark("secret"),
+			cty.NumberIntVal(11).Mark("secret"),
+		},
+		{ // Marked tuples return a marked length
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("hello"),
+				cty.StringVal("world"),
+			}).Mark("secret"),
+			cty.NumberIntVal(2).Mark("secret"),
+		},
+		{ // Marks on values in unmarked tuples do not propagate
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("hello").Mark("a"),
+				cty.StringVal("world").Mark("b"),
+			}),
+			cty.NumberIntVal(2),
+		},
+		{ // Marked objects return a marked length
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("hello"),
+				"b": cty.StringVal("world"),
+				"c": cty.StringVal("nice to meet you"),
+			}).Mark("secret"),
+			cty.NumberIntVal(3).Mark("secret"),
+		},
+		{ // Marks on object attribute values do not propagate
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("hello").Mark("a"),
+				"b": cty.StringVal("world").Mark("b"),
+				"c": cty.StringVal("nice to meet you").Mark("c"),
+			}),
+			cty.NumberIntVal(3),
+		},
 	}
 
 	for _, test := range tests {