The Copa del Rey 1921 was the 21st staging of the Copa del Rey, the Spanish football cup competition.

The competition started on 9 April 1921, and concluded on 8 May 1921, with the Final, held at the San Mamés in Bilbao, in which Athletic Bilbao lifted the trophy for the eighth time with a 4–1 victory over Athletic Madrid thanks to two braces from José María Laca and Domingo Acedo.

[1][2] FC Barcelona withdrew in protest after the Spanish Federation moved the final to Seville:[3] Athletic Madrid, who they had been drawn against, qualified for the semifinals.

Fortuna Vigo and Real Unión won one match each.

Sevilla FC won the semifinals, but was disqualified because of an illegal line-up in quarter-finals.