diff --git a/docs/public/static/error-codes.json b/docs/public/static/error-codes.json
index 7692a07d00909..077305b9019a4 100644
--- a/docs/public/static/error-codes.json
+++ b/docs/public/static/error-codes.json
@@ -288,5 +288,7 @@
   "288": "MUI X: useDisposable failed to detect React StrictMode.\nThe instance was disposed on StrictMode's simulated unmount and is about to be reused while torn down.\nThis is an internal invariant violation — please report it at https://github.com/mui/mui-x/issues.",
   "289": "MUI X Chat docs-correctness guard: could not load entry point %s (%s). The export surface cannot be enumerated.",
   "290": "MUI X Chat docs-correctness guard: %s has no module symbol (%s); it may be missing exports.",
-  "291": "MUI X Scheduler: All events must have a unique `id`.\nWithout an `id`, an event cannot be tracked and silently overwrites another event in the calendar state.\nAdd an `id` to every event, or set `eventModelStructure.id.getter` to derive one from your event model.\nAn event was provided without an `id`:\n%s"
+  "291": "MUI X Scheduler: All events must have a unique `id`.\nWithout an `id`, an event cannot be tracked and silently overwrites another event in the calendar state.\nAdd an `id` to every event, or set `eventModelStructure.id.getter` to derive one from your event model.\nAn event was provided without an `id`:\n%s",
+  "292": "MUI X Scheduler: Invalid FREQ value \"%s\". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.",
+  "293": "MUI X Scheduler: The recurrence rule must include a FREQ value. The frequency (DAILY, WEEKLY, MONTHLY, or YEARLY) is required for recurrence rules. Provide a freq value."
 }
diff --git a/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.test.ts b/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.test.ts
index 7d4b2dcef6758..a22f7e092595b 100644
--- a/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.test.ts
+++ b/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.test.ts
@@ -115,6 +115,46 @@ describe('recurring-events/rRuleString', () => {
       );
     });
 
+    it('should throw when FREQ has an unsupported value', () => {
+      expect(() => parseRRule(adapter, 'FREQ=HOURLY', 'default')).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "HOURLY". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
+    it('should throw when the object input has an unsupported freq value', () => {
+      expect(() =>
+        parseRRule(adapter, { freq: 'HOURLY' as SchedulerEventRecurrenceRule['freq'] }, 'default'),
+      ).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "HOURLY". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
+    it('should reject a non-canonical object freq instead of normalizing it', () => {
+      expect(() =>
+        parseRRule(adapter, { freq: 'daily' as SchedulerEventRecurrenceRule['freq'] }, 'default'),
+      ).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "daily". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
+    it('should throw a dedicated message when the object input has no freq value', () => {
+      expect(() => parseRRule(adapter, {} as SchedulerEventRecurrenceRule, 'default')).to.throw(
+        'MUI X Scheduler: The recurrence rule must include a FREQ value. The frequency (DAILY, WEEKLY, MONTHLY, or YEARLY) is required for recurrence rules. Provide a freq value.',
+      );
+    });
+
+    it('should throw when the object freq matches a prototype key', () => {
+      expect(() =>
+        parseRRule(
+          adapter,
+          { freq: 'toString' as SchedulerEventRecurrenceRule['freq'] },
+          'default',
+        ),
+      ).to.throw(
+        'MUI X Scheduler: Invalid FREQ value "toString". The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. Provide a supported frequency value.',
+      );
+    });
+
     it('should throw when the RRULE contains unsupported properties', () => {
       expect(() => parseRRule(adapter, 'FREQ=DAILY;FOO=bar', 'default')).to.throw(
         'MUI X Scheduler: Unsupported RRULE property "FOO". Supported properties are: FREQ, INTERVAL, BYDAY, BYMONTHDAY, BYMONTH, UNTIL, COUNT. Remove or replace the unsupported property.',
diff --git a/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.ts b/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.ts
index 39d7187c93ce1..f9d2faae5bf55 100644
--- a/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.ts
+++ b/packages/x-scheduler-internals-premium/src/internals/utils/recurring-events/rRuleString.ts
@@ -2,6 +2,7 @@ import { TemporalTimezone } from '@mui/x-scheduler-internals/base-ui-copy';
 import { Adapter } from '@mui/x-scheduler-internals/use-adapter';
 import {
   RecurringEventByDayValue,
+  RecurringEventFrequency,
   SchedulerProcessedEventRecurrenceRule,
   SchedulerEventRecurrenceRule,
 } from '@mui/x-scheduler-internals/models';
@@ -18,12 +19,40 @@ const SUPPORTED_RRULE_KEYS = new Set([
   'COUNT',
 ]);
 
+// Fails to compile if `RecurringEventFrequency` gains or loses a member, keeping the runtime check in sync with the type.
+const SUPPORTED_FREQUENCIES: Record<RecurringEventFrequency, true> = {
+  DAILY: true,
+  WEEKLY: true,
+  MONTHLY: true,
+  YEARLY: true,
+};
+
+function validateFreq(freq: string): RecurringEventFrequency {
+  if (!freq) {
+    throw new Error(
+      'MUI X Scheduler: The recurrence rule must include a FREQ value. ' +
+        'The frequency (DAILY, WEEKLY, MONTHLY, or YEARLY) is required for recurrence rules. ' +
+        'Provide a freq value.',
+    );
+  }
+  if (!Object.prototype.hasOwnProperty.call(SUPPORTED_FREQUENCIES, freq)) {
+    throw new Error(
+      `MUI X Scheduler: Invalid FREQ value "${freq}". ` +
+        'The frequency must be one of DAILY, WEEKLY, MONTHLY, or YEARLY. ' +
+        'Provide a supported frequency value.',
+    );
+  }
+  return freq as RecurringEventFrequency;
+}
+
 export function parseRRule(
   adapter: Adapter,
   input: string | SchedulerEventRecurrenceRule,
   timezone: TemporalTimezone,
 ): SchedulerProcessedEventRecurrenceRule {
   if (typeof input === 'object') {
+    // The typed object API validates `freq` as-is; unlike the RRULE string below, it is not normalized.
+    validateFreq(input.freq);
     if (input.until != null) {
       return {
         ...input,
@@ -72,7 +101,7 @@ export function parseRRule(
   }
 
   const rrule: SchedulerProcessedEventRecurrenceRule = {
-    freq: rruleObject.FREQ as SchedulerProcessedEventRecurrenceRule['freq'],
+    freq: validateFreq(rruleObject.FREQ),
   };
 
   if (rruleObject.INTERVAL) {