Gasquet, 22, the 23rd-ranked men's player, withdrew from his match at the Miami Masters against Spain's Albert Montanes because of a right shoulder injury and missed the just-concluded Estoril (Portugal) Open with a neck injury.
"The test of the B sample submitted at the end of March 2009, confirmed the positive result of the A sample taken on the same day," Gasquet said in a statement. "I want to prove my innocence and will explain myself at an appropriate time."
Gasquet reached the semifinals at Wimbledon in 2007, and has been ranked as high as No. 7 in his seven seasons on the men's tennis tour.
He could be suspended for two years, but said he is confident he will not receive a lengthy ban following tests by the International Tennis Federation.