Stein spent his first full season in the majors during the 1975 season. His season debut came on April 16, against the Texas Rangers, where in one at-bat he went hitless. In June, Stein was named the starting third baseman after Bill Melton, who was Chicago's regular third baseman, was benched for poor hitting. Stein also played back-up for second baseman Jorge Orta over the season. On July 20, in the second game of a doubleheader against the Milwaukee Brewers, Stein hit his first career grand slam. On the season, Stein batted .270 with 23 runs scored, 61 hits, seven doubles, one triple, three home runs, and 21 RBIs in 76 games played. In the field, he played 28 games at second base, 24 games at third base, and one game in left field. Stein also played 18 games that year at the designated hitter spot in the lineup.