diff --git a/lib/private/Calendar/CalendarEventBuilder.php b/lib/private/Calendar/CalendarEventBuilder.php
index 818a034911b16..68bff6fff7765 100644
--- a/lib/private/Calendar/CalendarEventBuilder.php
+++ b/lib/private/Calendar/CalendarEventBuilder.php
@@ -123,6 +123,21 @@ public function toIcs(): string {
 		foreach ($this->attendees as $attendee) {
 			self::addAttendeeToVEvent($vevent, 'ATTENDEE', $attendee);
 		}
+
+		// Prefix TZID values with '/' to reference globally-defined IANA timezone identifiers
+		// (RFC 5545 §3.2.19). This avoids the need for a VTIMEZONE component: without it,
+		// iTIP invite emails carry TZID=Europe/Berlin with no accompanying VTIMEZONE block,
+		// and some clients (e.g. Grommunio) misinterpret the time.
+		foreach (['DTSTART', 'DTEND'] as $propName) {
+			$prop = $vevent->$propName;
+			if ($prop !== null && isset($prop['TZID'])) {
+				$tzid = (string)$prop['TZID'];
+				if ($tzid !== '' && !str_starts_with($tzid, '/')) {
+					$prop['TZID'] = '/' . $tzid;
+				}
+			}
+		}
+
 		return $vcalendar->serialize();
 	}
 
diff --git a/tests/data/ics/event-builder-with-timezone.ics b/tests/data/ics/event-builder-with-timezone.ics
new file mode 100644
index 0000000000000..c662b766ac5cf
--- /dev/null
+++ b/tests/data/ics/event-builder-with-timezone.ics
@@ -0,0 +1,18 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Sabre//Sabre VObject 4.5.6//EN
+CALSCALE:GREGORIAN
+BEGIN:VEVENT
+UID:event-uid-123
+DTSTAMP:20250105T000000Z
+SUMMARY:My event
+DTSTART;TZID=/Europe/Berlin:20260511T110000
+DTEND;TZID=/Europe/Berlin:20260511T120000
+STATUS:CONFIRMED
+DESCRIPTION:Foo bar baz
+ORGANIZER;CN=Organizer;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:organizer@domain
+ .tld
+ATTENDEE;CN=Attendee;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION:mailto:atte
+ ndee@domain.tld
+END:VEVENT
+END:VCALENDAR
\ No newline at end of file
diff --git a/tests/data/ics/event-builder-with-timezone.ics.license b/tests/data/ics/event-builder-with-timezone.ics.license
new file mode 100644
index 0000000000000..23e2d6b19082e
--- /dev/null
+++ b/tests/data/ics/event-builder-with-timezone.ics.license
@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+SPDX-License-Identifier: AGPL-3.0-or-later
diff --git a/tests/lib/Calendar/CalendarEventBuilderTest.php b/tests/lib/Calendar/CalendarEventBuilderTest.php
index f4eacbfa85dc6..bf2a4a20ec576 100644
--- a/tests/lib/Calendar/CalendarEventBuilderTest.php
+++ b/tests/lib/Calendar/CalendarEventBuilderTest.php
@@ -98,6 +98,42 @@ public function testCreateInCalendar(): void {
 		$this->assertEquals('event-uid-123.ics', $actual);
 	}
 
+	public function testToIcsWithTimezone(): void {
+		$tz = new \DateTimeZone('Europe/Berlin');
+		$this->calendarEventBuilder->setStartDate(new DateTimeImmutable('2026-05-11T11:00:00', $tz));
+		$this->calendarEventBuilder->setEndDate(new DateTimeImmutable('2026-05-11T12:00:00', $tz));
+		$this->calendarEventBuilder->setStatus(CalendarEventStatus::CONFIRMED);
+		$this->calendarEventBuilder->setSummary('My event');
+		$this->calendarEventBuilder->setDescription('Foo bar baz');
+		$this->calendarEventBuilder->setOrganizer('organizer@domain.tld', 'Organizer');
+		$this->calendarEventBuilder->addAttendee('attendee@domain.tld', 'Attendee');
+
+		$actual = $this->calendarEventBuilder->toIcs();
+
+		// TZID must use the globally-defined form (RFC 5545 §3.2.19) so no VTIMEZONE is needed
+		$this->assertStringContainsString('DTSTART;TZID=/Europe/Berlin:', $actual);
+		$this->assertStringContainsString('DTEND;TZID=/Europe/Berlin:', $actual);
+		$this->assertStringNotContainsString('BEGIN:VTIMEZONE', $actual);
+
+		$expected = file_get_contents(\OC::$SERVERROOT . '/tests/data/ics/event-builder-with-timezone.ics');
+		$this->assertEquals($expected, $actual);
+	}
+
+	public function testToIcsWithUtcIsUnchanged(): void {
+		// UTC datetimes must stay as-is (Z suffix, no TZID parameter)
+		$this->calendarEventBuilder->setStartDate(new DateTimeImmutable('2026-05-11T09:00:00Z'));
+		$this->calendarEventBuilder->setEndDate(new DateTimeImmutable('2026-05-11T10:00:00Z'));
+		$this->calendarEventBuilder->setStatus(CalendarEventStatus::CONFIRMED);
+		$this->calendarEventBuilder->setSummary('My event');
+
+		$actual = $this->calendarEventBuilder->toIcs();
+
+		$this->assertStringContainsString('DTSTART:20260511T090000Z', $actual);
+		$this->assertStringContainsString('DTEND:20260511T100000Z', $actual);
+		$this->assertStringNotContainsString('TZID', $actual);
+		$this->assertStringNotContainsString('BEGIN:VTIMEZONE', $actual);
+	}
+
 	public function testToIcsWithoutStartDate(): void {
 		$this->calendarEventBuilder->setEndDate(new DateTimeImmutable('2025-01-05T17:19:58Z'));
 		$this->calendarEventBuilder->setSummary('My event');