Abu Said Uthman III (Arabic: عثمان الثاني المريني) (Abu Said Uthman ibn Abi l-Abbas ibn Abi Salim), (1383 – 21 October 1420) was Marinid ruler of Morocco from 19 March 1398 to 1420, the last effective ruler of that dynasty.

Yusuf III's son Ahmad laid siege to Gibraltar and defeated several Moroccan attempts to break out.

The Portuguese gathered a fleet under the command of princes Henry the Navigator and John of Reguengos to relieve Ceuta.

Opportunistically, the Nasrid rulers of Granada and the Abdalwadids of the Kingdom of Tlemcen intervened, each sponsoring different candidates for the Moroccan throne.

[6][4] Hearing the news of the sultan's assassination, Abu Zakariya hurried from Salé and seized control of the royal palace of Fez, proclaiming the orphan child Abd al-Haqq as the new Marinid sultan and appointing himself his regent and chief minister (vizier).

Granadan and Tlemcen interventions and intrigues continued, regional governors seized control of their districts, selling and re-selling their allegiance to the highest bidder, Sufi-inspired religious radicals drummed up mobs to seize control of urban centers and take to the field, while rowdy rural nomads, the Hilalian Bedouin tribesmen, availed themselves of the general breakdown of law and order to launch a series of bandit raids on smaller towns and settlements.