Darnold made the expected announcement in an Instagram video Wednesday.
About 25 minutes earlier, UCLA quarterback Josh Rosen announced he will enter the draft. Both local products are expected to be high first-round selections in April.
Darnold took over as the Trojans' starting quarterback four games into last season as a redshirt freshman. He won the Rose Bowl in his first season with a record 453-yard, five-touchdown performance against Penn State, and he led the Trojans to their first Pac-12 championship since 2008 this season.
Darnold passed for 7,229 yards with 57 touchdown passes and 22 interceptions in 27 games behind center for the Trojans.
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