Mon Rivera is the common name given to two distinct Puerto Rican musicians (both born in Mayagüez), namely Monserrate Rivera Alers (originally nicknamed Rate, later referred to as "Don Mon", or Mon The Elder, and sometimes erroneously credited as Ramón in songwriting credits) and his oldest son, Efraín Rivera Castillo (1924-1978), (referred to early in his career as "Moncito", or Little Mon, and later known by his father's moniker). This article refers mainly to Efraín, a popular band leader known in salsa, plena and Latin jazz circles.

Efraín was specifically known for salsa and a Puerto Rican style called plena. He is credited for a fast humorous style and for introducing the sound of an all-trombone brass section to Afro-Rican orchestra music.