Castro was acquired by Miami from the New York Yankees in the Giancarlo Stanton trade in December. He does not want to be part of the Marlins' rebuilding effort after enduring losing in the beginning of his career with the Cubs, according to The Athletic, which first reported the news.
He becomes the third current Marlin to request a trade, joining outfielder Christian Yelich and catcher J.T. Realmuto, after the new Marlins ownership, fronted by Bruce Sherman and CEO Derek Jeter, traded away stars Stanton, Marcell Ozuna and Dee Gordon.
Starlin is a four-time All Star, including last season when he hit .300 with 16 home runs and 63 RBIs for the Yankees.
If he is not traded, Castro is expected to start at second base for the Marlins, replacing Gordon, who was traded to the Seattle Mariners this offseason.
He has played second base the past two seasons after beginning his career as a shortstop with the Cubs.
Castro, 27, has a career batting average of .282 with 99 home runs and 496 RBIs.