Islam recognizes three mosques of supreme spiritual significance, though only two are located in Saudi Arabia. The third, often included in this trio, sits in Jerusalem.
Encompassing the Kaaba, Islam's most sacred site, this mosque is the focal point of the Hajj pilgrimage and the direction (qibla) for Muslim prayers worldwide. Its vast complex can accommodate millions of worshippers.
Built by the Prophet Muhammad himself, this mosque houses his tomb beneath the iconic Green Dome. It is the second holiest site in Islam and a major pilgrimage destination.
Located in the Old City of Jerusalem, Al-Aqsa is the third holiest site in Islam. It is believed to be the "Farthest Mosque" referenced in the Quran's Night Journey (Isra and Mi'raj). While not in Saudi Arabia, its inclusion in the trio reflects its profound theological importance.
The question's framing—"in Saudi Arabia"—is slightly inaccurate. Only the first two mosques are Saudi; Al-Aqsa lies in Jerusalem, a city of deep significance to Muslims, Christians, and Jews alike.