diff --git a/Cargo.lock b/Cargo.lock
index 302d30fd9..d018aa027 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6589,6 +6589,7 @@ name = "vectorcraft-js"
 version = "0.24.0"
 dependencies = [
  "chrono",
+ "getrandom 0.4.3",
  "proptest",
  "unicode-general-category",
 ]
diff --git a/crates/js/Cargo.toml b/crates/js/Cargo.toml
index 5848c1a91..610c8f9bd 100644
--- a/crates/js/Cargo.toml
+++ b/crates/js/Cargo.toml
@@ -12,6 +12,12 @@ workspace = true
 [dependencies]
 chrono = { workspace = true }
 unicode-general-category = "1"
+# The seed of Math.random (interp.rs).
+getrandom = "0.4"
+
+[target.'cfg(all(target_arch = "wasm32", target_os = "unknown"))'.dependencies]
+# The browser's crypto.getRandomValues.
+getrandom = { version = "0.4", features = ["wasm_js"] }
 
 [dev-dependencies]
 proptest = { workspace = true }
diff --git a/crates/js/src/builtins_tests.rs b/crates/js/src/builtins_tests.rs
index e0593d1dd..9d917ae32 100644
--- a/crates/js/src/builtins_tests.rs
+++ b/crates/js/src/builtins_tests.rs
@@ -605,6 +605,17 @@ fn math_functions_follow_the_special_cases() {
     truth("var n = 0; Math.max({valueOf: function () { n++; return NaN; }}, {valueOf: function () { n++; return 1; }}); n === 2");
 }
 
+#[test]
+fn math_random_is_seeded_from_the_os_random_source() {
+    // On wasm32-unknown-unknown `RandomState`'s keys come from memory addresses, which are the
+    // same on every page load: the seed must come from the OS (the browser's crypto) instead.
+    let src = include_str!("interp.rs");
+    let Some(start) = src.find("fn random_seed()") else { panic!("interp.rs has no random_seed") };
+    let body = &src[start..];
+    let body = &body[..body.find("\n}").unwrap_or(body.len())];
+    assert!(body.contains("getrandom::u64()"), "Math.random isn't seeded from the OS random source");
+}
+
 // ---- JSON ----
 
 #[test]
diff --git a/crates/js/src/interp.rs b/crates/js/src/interp.rs
index 64204423f..5d59d92d2 100644
--- a/crates/js/src/interp.rs
+++ b/crates/js/src/interp.rs
@@ -307,13 +307,16 @@ fn index_keys_bytes(len: usize) -> usize {
     total
 }
 
-/// A seed for `Math.random` that differs from realm to realm, from the standard library's
-/// randomly keyed hasher; never zero.
+/// A seed for `Math.random` that differs from realm to realm and from run to run, from the OS
+/// random source (the browser's crypto on the web), or the standard library's randomly keyed
+/// hasher when that fails; never zero.
 fn random_seed() -> u64 {
     use std::hash::{BuildHasher, Hasher};
-    let mut h = std::collections::hash_map::RandomState::new().build_hasher();
-    h.write_u64(0x9e37_79b9_7f4a_7c15);
-    h.finish() | 1
+    getrandom::u64().unwrap_or_else(|_| {
+        let mut h = std::collections::hash_map::RandomState::new().build_hasher();
+        h.write_u64(0x9e37_79b9_7f4a_7c15);
+        h.finish()
+    }) | 1
 }
 
 /// The estimated bytes of a binding or property named by `len` code units.