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 said 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 messages.
Curtis played for the then-California Angels, Detroit Tigers, Los Angeles Dodgers, Cleveland Indians, New York Yankees and Texas Rangers from 1992 to 2001. He won two World Series rings with the Yankees.