Yasiin Bey (/jæˈsiːn ˈbeɪ/; born Dante Terrell Smith; December 11, 1973), better known by his former stage name Mos Def (/ˈmoʊs ˈdɛf/), is an American hip hop recording artist, actor, comedian, and activist from Brooklyn, New York City, New York. Best known for his music, Mos Def embarked on his hip hop career in 1994, alongside his siblings in the short-lived rap group Urban Thermo Dynamics (UTD), after which he appeared on albums by Da Bush Babees and De La Soul. He subsequently formed the duo Black Star, alongside fellow Brooklyn-based rapper Talib Kweli, and released their eponymous debut album in 1998. He was a major force in late-1990s underground hip hop while under Rawkus Records. In 1999, Mos Def released his solo debut, Black on Both Sides, under Rawkus and Priority Records. His debut was followed by The New Danger (2004), True Magic (2006) and The Ecstatic (2009).

Prior to his career in music, Mos Def first entered public light as a child actor, having played roles in television movies, sitcoms, and theater, some of which were under the name Dante Beze. At the age of 14, he appeared in the TV movie God Bless the Child, which aired on ABC in 1988. He played the oldest child in the 1990 family sitcom You Take the Kids, shortly before it was cancelled. In 1995, he played the character "Dante" in The Cosby Mysteries. Since the early 2000s, Mos Def is well known for his roles in films such as Something the Lord Made, Next Day Air, The Hitchhiker's Guide to the Galaxy, 16 Blocks, Be Kind Rewind, The Italian Job, Bamboozled and Brown Sugar, as well as for his portrayal of Brother Sam in the Showtime drama series, Dexter. He is also known as the host of Def Poetry Jam, which aired on HBO between 2002 and 2007.

Mos Def has been vocal on several social and political causes, including police brutality, the idea of American exceptionalism, and the state of black Americans.