A two-speed transmission alternately establishes two speed ratios between a primary drive pulley and one or more accessories pulleys of an accessory drive system. The two-speed transmission includes concentric input and output shafts fixed to input and output pulleys respectively. When an actuator is disengaged, a one-way clutch transfers power between the input shaft and the output shaft. The input shaft is fixed to the planet carrier of a planetary gear set and the output shaft is fixed to the sun gear of the planetary gear set. When the actuator is engaged, the ring gear of the planetary gear set is held establishing an overdrive speed relationship between the input pulley and the output pulley.