diff --git a/crates/compiler/builtins/roc/Str.roc b/crates/compiler/builtins/roc/Str.roc
index d789a01353..f79973f922 100644
--- a/crates/compiler/builtins/roc/Str.roc
+++ b/crates/compiler/builtins/roc/Str.roc
@@ -44,6 +44,7 @@ interface Str
         walkScalars,
         walkScalarsUntil,
         withCapacity,
+        withPrefix,
     ]
     imports [
         Bool.{ Bool },
@@ -565,3 +566,7 @@ strToNumHelp = \string ->
         Ok result.aresult
     else
         Err InvalidNumStr
+
+## Adds the specified prefix to the string, like a reversed Str.concat
+withPrefix : Str, Str -> Str
+withPrefix = \str, prefix -> Str.concat prefix str
diff --git a/crates/compiler/module/src/symbol.rs b/crates/compiler/module/src/symbol.rs
index 50712f1764..b78549de60 100644
--- a/crates/compiler/module/src/symbol.rs
+++ b/crates/compiler/module/src/symbol.rs
@@ -1297,6 +1297,7 @@ define_builtins! {
         51 STR_REPLACE_FIRST: "replaceFirst"
         52 STR_REPLACE_LAST: "replaceLast"
         53 STR_WITH_CAPACITY: "withCapacity"
+        54 STR_WITH_PREFIX: "withPrefix"
     }
     6 LIST: "List" => {
         0 LIST_LIST: "List" exposed_apply_type=true // the List.List type alias
diff --git a/crates/compiler/test_gen/src/gen_str.rs b/crates/compiler/test_gen/src/gen_str.rs
index d38e8e0fcb..c1bd9385f3 100644
--- a/crates/compiler/test_gen/src/gen_str.rs
+++ b/crates/compiler/test_gen/src/gen_str.rs
@@ -1958,3 +1958,27 @@ fn with_capacity_concat() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn str_with_prefix() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            Str.withPrefix "world!" "Hello "
+            "#
+        ),
+        RocStr::from("Hello world!"),
+        RocStr
+    );
+
+    assert_evals_to!(
+        indoc!(
+            r#"
+            "two" |> Str.withPrefix "Forty "
+            "#
+        ),
+        RocStr::from("Forty two"),
+        RocStr
+    );
+}