diff --git a/packages/date-time-picker/src/vaadin-date-time-picker.js b/packages/date-time-picker/src/vaadin-date-time-picker.js
index 9fff9adfb3..f0805914c7 100644
--- a/packages/date-time-picker/src/vaadin-date-time-picker.js
+++ b/packages/date-time-picker/src/vaadin-date-time-picker.js
@@ -724,6 +724,12 @@ class DateTimePicker extends FieldMixin(DisabledMixin(FocusMixin(ThemableMixin(E
     if (timePicker) {
       timePicker.required = required;
     }
+
+    if (this.__oldRequired && !required) {
+      this.validate();
+    }
+
+    this.__oldRequired = required;
   }
 
   /** @private */
diff --git a/packages/date-time-picker/test/validation.test.js b/packages/date-time-picker/test/validation.test.js
index ffca0be55f..28e81e18b4 100644
--- a/packages/date-time-picker/test/validation.test.js
+++ b/packages/date-time-picker/test/validation.test.js
@@ -200,6 +200,11 @@ const fixtures = {
         dateTimePicker.validate();
         expect(dateTimePicker.invalid).to.be.true;
       });
+
+      it('should validate when setting required to false', () => {
+        dateTimePicker.required = false;
+        expect(validateSpy).to.be.calledOnce;
+      });
     });
 
     describe('document losing focus', () => {