diff --git a/src/builtin/string.rs b/src/builtin/string.rs
index b77efcf..179dd19 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/string.rs
@@ -146,4 +146,33 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             })
         }),
     );
+    f.insert(
+        "str-insert".to_owned(),
+        Box::new(|args, _| {
+            max_args!(args, 3);
+            let (mut string, q) = match arg!(args, 0, "string") {
+                Value::Ident(i, q) => (i, q),
+                v => return Err(format!("$string: {} is not a string.", v).into()),
+            };
+
+            let substr = match arg!(args, 1, "insert") {
+                Value::Ident(i, _) => i,
+                v => return Err(format!("$insert: {} is not a string.", v).into()),
+            };
+
+            let index = match arg!(args, 2, "index") {
+                Value::Dimension(n, _) => n.to_integer().to_usize().unwrap(),
+                v => return Err(format!("$index: {} is not a number.", v).into()),
+            };
+
+            let quotes = match q {
+                QuoteKind::Double | QuoteKind::Single => QuoteKind::Double,
+                QuoteKind::None => QuoteKind::None,
+            };
+
+            string.insert_str(index - 1, &substr);
+
+            Ok(Value::Ident(string, quotes))
+        }),
+    );
 }
diff --git a/tests/strings.rs b/tests/strings.rs
index 7f81ee4..f4e7567 100644
--- a/tests/strings.rs
+++ b/tests/strings.rs
@@ -114,3 +114,33 @@ test!(
     "a {\n  color: 1;\n}\n"
 );
 test!(str_index_null, "a {\n  color: str-index(abcd, X);\n}\n", "");
+test!(
+    str_insert_start,
+    "a {\n  color: str-insert(\"abcd\", \"X\", 1);\n}\n",
+    "a {\n  color: \"Xabcd\";\n}\n"
+);
+test!(
+    str_insert_middle,
+    "a {\n  color: str-insert(\"abcd\", \"X\", 4);\n}\n",
+    "a {\n  color: \"abcXd\";\n}\n"
+);
+test!(
+    str_insert_end,
+    "a {\n  color: str-insert(\"abcd\", \"X\", 5);\n}\n",
+    "a {\n  color: \"abcdX\";\n}\n"
+);
+test!(
+    str_insert_sgl_quotes,
+    "a {\n  color: str-insert('abcd', \"X\", 4);\n}\n",
+    "a {\n  color: \"abcXd\";\n}\n"
+);
+test!(
+    str_insert_no_quotes,
+    "a {\n  color: str-insert(abcd, \"X\", 4);\n}\n",
+    "a {\n  color: abcXd;\n}\n"
+);
+test!(
+    str_insert_empty_string,
+    "a {\n  color: str-insert(abcd, \"\", 4);\n}\n",
+    "a {\n  color: abcd;\n}\n"
+);