diff --git a/changelog.d/8460-array-join-capacity.md b/changelog.d/8460-array-join-capacity.md
new file mode 100644
index 0000000000..95519402b8
--- /dev/null
+++ b/changelog.d/8460-array-join-capacity.md
@@ -0,0 +1,5 @@
+### Performance
+
+- Preallocate `Array.prototype.join`'s exact separator-byte floor instead of
+  growing its Rust assembly buffer from zero capacity. Element coercion remains
+  single-pass, so user `toString` side effects are not duplicated.
diff --git a/crates/perry-runtime/src/array/iter_methods.rs b/crates/perry-runtime/src/array/iter_methods.rs
index db3e02f247..db5f5a6af8 100644
--- a/crates/perry-runtime/src/array/iter_methods.rs
+++ b/crates/perry-runtime/src/array/iter_methods.rs
@@ -1118,8 +1118,13 @@ pub extern "C" fn js_array_join(
             std::str::from_utf8_unchecked(std::slice::from_raw_parts(sep_data, sep_len))
         };
 
-        // Build result string
-        let mut result = String::new();
+        // Separators are an exact lower bound for the result. Reserving them
+        // up front avoids the zero-capacity growth ladder without speculatively
+        // coercing elements (which could run user code or mutate the array).
+        let separator_bytes = sep_str
+            .len()
+            .saturating_mul(length.saturating_sub(1) as usize);
+        let mut result = String::with_capacity(separator_bytes);
         for i in 0..length as usize {
             if i > 0 {
                 result.push_str(sep_str);
diff --git a/test-files/test_issue_8434_array_join_capacity.ts b/test-files/test_issue_8434_array_join_capacity.ts
new file mode 100644
index 0000000000..c6ba300174
--- /dev/null
+++ b/test-files/test_issue_8434_array_join_capacity.ts
@@ -0,0 +1,23 @@
+const show = (label: string, value: string): void => {
+  console.log(`${label}:${value}|${value.length}`);
+};
+
+show("holes", new Array(3).join("|"));
+show("empty", ["", "", ""].join(""));
+show("unicode", ["A", "😀", "é"].join("·"));
+
+let calls = 0;
+const values: unknown[] = [];
+const allocating = {
+  toString(): string {
+    calls++;
+    for (let i = 0; i < 128; i++) {
+      ("allocation-" + i).repeat(32);
+    }
+    values[1] = "after";
+    return "before";
+  },
+};
+values.push(allocating, "original", "tail");
+show("coerce", values.join("/"));
+console.log(`calls:${calls}`);