St. Marys advances to the Class AA semifinals where it will face No. 2 Charleston Catholic on Thursday.
The Blue Devils led wire-to-wire, taking a double-digit lead in the first quarter, holding off a rally and cruising to the finish line.
Addie Davis (17), Brynnley Bulluck (16) and Laney Watkins (10) all scored in double figures for St. Marys.
Elsa Law led Ritchie County with 27 points in the loss.
