"It's time to take the next step," said Martin, who is projected as a top-10 pick.
Martin's college career ended with a loss, as the Cardinal were edged by Oklahoma State in the Tostitos Fiesta Bowl on Monday. "This one hurt," Martin said of the loss, "but I want a new challenge."
Asked what he expects to bring to the NFL, Martin said "Hard work. I pride myself on competing to be the hardest worker on the field."
Martin, listed at 6-foot-6 and 304 pounds, is No. 9 overall on the latest Big Board posted by ESPN NFL draft analyst Mel Kiper Jr. A Los Angeles native who attended Stanford out of Harvard-Westlake School, he's rated No. 6 overall and No. 2 at his position by ESPN Scouts Inc.
He was a second-team AP All-American selection this season as part of an offensive line that protected quarterback Andrew Luck, who is widely expected to be selected No. 1 overall in the NFL draft.
DeCastro, a first-team All-American, also told Schad it was "time" to move on.
"It's the right thing to do. I had an awesome time at Stanford and I wouldn't trade the memories for anything," DeCastro said.
The 6-foot-4½, 313-pound DeCastro is ranked 15th by Scouts Inc. and is No. 14 on Kiper's Big Board.
As one of the most promising and productive periods in the program's history ends, the real challenge for first-year head coach David Shaw begins this offseason: Keeping the Cardinal among the nation's elite.
Along with Luck, Martin and DeCastro, much of the supporting cast on both sides of the ball is graduating, including top tight end Coby Fleener and starting safeties Michael Thomas and Delano Howell. Junior linebacker Chase Thomas is still deciding whether to return.
Martin said he's proud of his role in building the program.
"We built the foundation, now it's time for the team to make the national championship," he said.
ESPN college football reporter Joe Schad and The Associated Press contributed to this report.