The 17-year-old, who has been a prolific scorer for the Pirates youth team, has penned a two-year deal.
Rovers manager Paul Trollope told the club's official website: "We are pleased to have signed Lamar on a pro deal.
"He's a player who came to our attention last year, when he was in the U-16 squad, but he's come up through our Centre of Excellence.
"I think it shows what we are trying to achieve at the club, and signing Lamar is another step in the right direction."
Powell added: "Naturally I am delighted to have signed my first pro contract for the club and I'm looking forward to making further progress over the next two years."