The 24-year-old arrived from Saracens in late November, having scored eight tries in 14 games for the London club.
Sporting general manager Neil McIlroy said: “Samson has had to be very patient but he has taken his opportunities, and I’m delighted he will be with us next season.
“He is an explosive athlete who is very professional in his preparation, and I look forward to seeing his continued progress with us now that we have secured him for another season.”
Adejimi, who has played twice for the Red Bulls, said: “I’m absolutely over the moon to have re-signed for next season.
“It’s an extremely exciting project, which I’m looking forward to playing a part in.
“There’s a great bunch of lads here who want to take the club in the right direction, and the staff genuinely care here too, which is the making of a great environment.
“I’m looking forward to ripping in for the remainder of the season and then seeing where we can get to next year.”