The England under-21 forward sent an angry tweet when Albion prevented him moving to Spurs at the start of the season, with his club rejecting four bids from the Londoners.
Since then Berahino has endured a frustrating campaign and has been used mainly as a substitute.
"It is something I look back on and I really regret. I should never have said that," the 22-year-old explained in a video on Albion's YouTube channel.
"I hold my hands up and I say it was a mistake from me and I just apologise to all the fans out there that have always supported me and also the club that has always believed in me."
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After failing to get his summer move, Berahino was tipped to leave Albion in January but the club rejected a 21 million pounds ($30.5 million) bid from Newcastle United late in the transfer window.
This time there has been no angry fallout and the player, who has come through the club's academy to become their top scorer in the last two seasons, said he was focussing on regaining peak fitness.
"I have missed a lot of football and I am just trying to get myself back into match fitness and sharper again like I was last season," he said.
(Reporting by Toby Davis; Editing by Tony Jimenez)
