diff --git a/test/intl402/NumberFormat/prototype/format/intl-mathematical-value-extremes.js b/test/intl402/NumberFormat/prototype/format/intl-mathematical-value-extremes.js
new file mode 100644
index 00000000000..95a2a6f0d34
--- /dev/null
+++ b/test/intl402/NumberFormat/prototype/format/intl-mathematical-value-extremes.js
@@ -0,0 +1,48 @@
+// Copyright (C) 2026 dmvjs. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-tointlmathematicalvalue
+description: >
+  ToIntlMathematicalValue clamps string values whose mathematical value rounds
+  to zero or infinity as a Number.
+info: |
+  ToIntlMathematicalValue ( value )
+
+  ...
+  9. If intlMV is a mathematical value, then
+     a. Let rounded be RoundMVResult(abs(intlMV)).
+     b. If rounded is +∞𝔽 and intlMV < 0, return ~negative-infinity~.
+     c. If rounded is +∞𝔽, return ~positive-infinity~.
+     d. If rounded is +0𝔽 and intlMV < 0, return ~negative-zero~.
+     e. If rounded is +0𝔽, return 0.
+features: [Intl.NumberFormat-v3]
+locale: [en-US]
+---*/
+
+// maximumSignificantDigits: 1 is used to expose incorrect preservation of the
+// exact mathematical value in engines that do not apply step 9 clamping.
+var nfSig = new Intl.NumberFormat("en-US", { maximumSignificantDigits: 1 });
+var nf = new Intl.NumberFormat("en-US");
+
+// Values too small: MV rounds to +0 (step 9e) — must format as "0".
+assert.sameValue(nfSig.format("1e-1000"), "0", "1e-1000");
+assert.sameValue(nfSig.format("2e-324"),  "0", "2e-324 (largest inadmissible positive)");
+
+// Values too small, negative: MV rounds to -0 (step 9d).
+assert.sameValue(nfSig.format("-1e-1000"), nfSig.format(-0), "-1e-1000");
+assert.sameValue(nfSig.format("-2e-324"),  nfSig.format(-0), "-2e-324 (largest inadmissible negative)");
+
+// Values too large: MV rounds to +∞ (step 9c) — must format as ∞.
+assert.sameValue(nf.format("1e1000"),                  nf.format(Infinity), "1e1000");
+assert.sameValue(nf.format("1.7976931348623159e308"),  nf.format(Infinity), "1.7976931348623159e308 (smallest inadmissible large positive)");
+
+// Values too large, negative: MV rounds to -∞ (step 9b).
+assert.sameValue(nf.format("-1e1000"),                 nf.format(-Infinity), "-1e1000");
+assert.sameValue(nf.format("-1.7976931348623159e308"), nf.format(-Infinity), "-1.7976931348623159e308 (smallest inadmissible large negative)");
+
+// Admissible boundary values: must not be clamped to zero or infinity.
+assert.notSameValue(nfSig.format("3e-324"),              "0",                "3e-324 (smallest admissible positive)");
+assert.notSameValue(nfSig.format("-3e-324"),             nfSig.format(-0),   "-3e-324 (smallest admissible negative)");
+assert.notSameValue(nf.format("1.7976931348623158e308"), nf.format(Infinity),"1.7976931348623158e308 (largest admissible positive)");
+assert.notSameValue(nf.format("-1.7976931348623158e308"),nf.format(-Infinity),"-1.7976931348623158e308 (largest admissible negative)");