diff --git a/src/lib/isISO8601.js b/src/lib/isISO8601.js
index 6eea9ae27..5a6780ef8 100644
--- a/src/lib/isISO8601.js
+++ b/src/lib/isISO8601.js
@@ -2,9 +2,11 @@ import assertString from './util/assertString';
 
 /* eslint-disable max-len */
 // from http://goo.gl/0ejHHW
-const iso8601 = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W(0[1-9]|[1-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[0-6])))([T\s]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
+// Handle hour 24 separately: all lower-order components must be zero.
+// The (?!24) guard prevents the optional-hour branch from reading 24 as seconds.
+const iso8601 = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W(0[1-9]|[1-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[0-6])))([T\s](?:(?!24)((([01]\d|2[0-3])((:?)[0-5]\d)?)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?|24(?:00(?:00)?|:00(?::00)?)?(?:[.,]0+)?)([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
 // same as above, except with a strict 'T' separator between date and time
-const iso8601StrictSeparator = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W(0[1-9]|[1-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[0-6])))([T]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
+const iso8601StrictSeparator = /^([\+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W(0[1-9]|[1-4]\d|5[0-3])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[0-6])))([T](?:(?!24)((([01]\d|2[0-3])((:?)[0-5]\d)?)([\.,]\d+(?!:))?)?(\17[0-5]\d([\.,]\d+)?)?|24(?:00(?:00)?|:00(?::00)?)?(?:[.,]0+)?)([zZ]|([\+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/;
 /* eslint-enable max-len */
 const isValidDate = (str) => {
   // str must have passed the ISO8601 check
diff --git a/test/validators.test.js b/test/validators.test.js
index 98d2a12ff..492f1f48a 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -12420,6 +12420,66 @@ describe('Validators', () => {
     });
   });
 
+  [{}, { strict: true }, { strictSeparator: true }, { strict: true, strictSeparator: true }]
+    .forEach((options) => {
+      it(`should validate ISO 8601 end-of-day times with ${JSON.stringify(options)}`, () => {
+        test({
+          validator: 'isISO8601',
+          args: [options],
+          valid: [
+            '2009-01-01T24',
+            '2009-01-01T24.0',
+            '2009-01-01T2400',
+            '2009-01-01T24:00',
+            '2009-01-01T240000',
+            '2009-01-01T24:00:00',
+            '2009-01-01T2400,000',
+            '2009-01-01T24:00.000',
+            '2009-01-01T240000.000',
+            '2009-01-01T24:00:00,000',
+            '2009-01-01T24:00:00Z',
+            '2009-01-01T240000+0530',
+            '2009-01-01T24:00:00.000-05:30',
+            '2009-01-01T24:00:00-01',
+          ],
+        });
+      });
+
+      it(`should reject invalid ISO 8601 end-of-day times with ${JSON.stringify(options)}`, () => {
+        test({
+          validator: 'isISO8601',
+          args: [options],
+          invalid: [
+            '2009-01-01T24:0000',
+            '2009-01-01T2400:00',
+            '2009-01-01T24:0030',
+            '2009-01-01T240030',
+            '2009-01-01T24:01',
+            '2009-01-01T2401',
+            '2009-01-01T24:00:01',
+            '2009-01-01T240001',
+            '2009-01-01T24.1',
+            '2009-01-01T24:00.0001',
+            '2009-01-01T2400,0001',
+            '2009-01-01T24:00:00.001Z',
+            '2009-01-01T240000.1+0530',
+            '2009-01-01T24:00.0:00',
+            '2009-01-01T24:00:00+24:00',
+          ],
+        });
+      });
+
+      it(`should preserve ISO 8601 date-time separator handling with ${JSON.stringify(options)}`, () => {
+        test({
+          validator: 'isISO8601',
+          args: [options],
+          [options.strictSeparator ? 'invalid' : 'valid']: [
+            '2009-01-01 24:00:00',
+          ],
+        });
+      });
+    });
+
   it('should validate ISO 8601 dates, with strict = true (regression)', () => {
     test({
       validator: 'isISO8601',