"I've been at 'SC for four years and have had a pretty productive career," Grimble told the LA Times. "I'm healthy and just want to move on and give it a shot…. I feel like it was kind of my time."
Grimble makes for the fifth player who will leave Troy with eligibility remaining.
The 6-foot-5 250-pound tight end told SCPlaybook during bowl practices that he in fact "planned" to return to USC for his final season.
"I've thought about it a little bit," Grimble answered at the time, when asked if he'd come to a decision about his future. "It's more of a personal decision for me, I feel like with everything that's happened this season that I think it will be best for me to stay in school.
"So that's probably what I'm going to be doing this year."
Draft evaluation papers were not requested by Grimble.
It remains unclear what prompted Grimble from planning to stay at USC to change his mind and announce for the NFL.
Grimble, who was slowed by nagging injuries throughout the season, finished the year with 25 receptions for 271 yards, including two touchdowns.
Fellow redshirt junior tight end Randall Telfer already announced he would return for his final season.
USC now has only two scholarship tight ends on its roster, including Jalen Cope-Fitzpatrick. Sarkisian recently picked up a commitment from 2014 Ventura (Calif.) St. Bonaventure tight end Bryce Dixon.