Curtis, 44, was convicted of six counts of criminal sexual conduct and will be sentenced Sept. 26 in Barry County.
Curtis was accused of touching two 15-year-old girls last year when he was a volunteer weight-room strength trainer at an area high school. A 16-year-old girl later told authorities that he touched and fondled her in 2011.
"We give these young ladies so much credit for their courage," Barry County Prosecutor Julie Nakfoor-Pratt told MLive.com after the verdict. "It took a lot of courage to testify. This has been such a long road for them. This is a step in the healing process for them."
Curtis' attorney, David Dodge, did not immediately return email and phone messages from The Associated Press seeking comment Friday evening.
Between 1992 and 2001, Curtis played for the then-California Angels, Detroit Tigers, Los Angeles Dodgers, Cleveland Indians, New York Yankees and Texas Rangers. He won two World Series rings with the Yankees.