diff --git a/changelog.d/fixed-557-nps-relay-register-ip-slot.md b/changelog.d/fixed-557-nps-relay-register-ip-slot.md
new file mode 100644
index 0000000..9b0b1cc
--- /dev/null
+++ b/changelog.d/fixed-557-nps-relay-register-ip-slot.md
@@ -0,0 +1,2 @@
+- The NPS relay no longer consumes the per-IP registration slot when the daily
+  registration cap refuses a registration (hivecommons/docs#557).
diff --git a/netlify/nps-relay/relay.test.ts b/netlify/nps-relay/relay.test.ts
index 5eacb18..b0fa620 100644
--- a/netlify/nps-relay/relay.test.ts
+++ b/netlify/nps-relay/relay.test.ts
@@ -37,6 +37,7 @@ import {
   PURPOSE_SUBMIT,
   RATE_WINDOW_MS,
   REGISTER_DAY_PREFIX,
+  REGISTER_IP_RATE_PREFIX,
   REGISTER_PATH,
   RELAY_BASE_PATH,
   SIGNATURE_MAX_SKEW_MS,
@@ -488,6 +489,20 @@ describe("POST /api/nps/register", () => {
     // An already-registered install can still re-register idempotently.
     expect((await handleRelayRequest(register(), deps())).status).toBe(200);
   });
+
+  it("releases the per-IP slot when the day cap refuses the registration", async () => {
+    const dayKey = `${REGISTER_DAY_PREFIX}${new Date(now).toISOString().slice(0, 10)}`;
+    store.data.set(dayKey, { count: MAX_REGISTRATIONS_PER_DAY });
+    const res = await handleRelayRequest(register(), deps());
+    expect(res.status).toBe(429);
+    expect(store.data.has(`${INSTALL_PREFIX}${hive.installId}`)).toBe(false);
+    const ipRecords = [...store.data.entries()].filter(([k]) =>
+      k.startsWith(REGISTER_IP_RATE_PREFIX)
+    );
+    for (const [, rec] of ipRecords) {
+      expect((rec as { times: number[] }).times).toEqual([]);
+    }
+  });
 });
 
 describe("POST /api/nps (submit)", () => {
diff --git a/netlify/nps-relay/relay.ts b/netlify/nps-relay/relay.ts
index 3b50301..57bcf1c 100644
--- a/netlify/nps-relay/relay.ts
+++ b/netlify/nps-relay/relay.ts
@@ -743,10 +743,13 @@ async function handleRegister(
   }
 
   // Reserve the per-IP and per-day slots before creating the install, so a
-  // limit that is hit never leaves a registration behind.
+  // limit that is hit never leaves a registration behind. The two writes are
+  // not atomic together: if the day cap refuses, the IP slot is released so a
+  // rejected registration does not consume the independent IP quota.
+  const ipKey = `${REGISTER_IP_RATE_PREFIX}${ipHash(deps)}`;
   const ipDenied = await reserveSlot(
     store,
-    `${REGISTER_IP_RATE_PREFIX}${ipHash(deps)}`,
+    ipKey,
     MAX_REGISTRATIONS_PER_IP_PER_WINDOW,
     now
   );
@@ -760,7 +763,10 @@ async function handleRegister(
     if (count >= MAX_REGISTRATIONS_PER_DAY) return { reject: RATE_LIMITED() };
     return { next: { count: count + 1 } satisfies DayCounter };
   });
-  if (dayDenied) return dayDenied;
+  if (dayDenied) {
+    await releaseSlot(store, ipKey, now);
+    return dayDenied;
+  }
 
   const record: InstallRecord = {
     public_key: reg.public_key,