We experimentally demonstrate a 4 × 4 nonblocking silicon thermo-optic (TO) switch fabric consisting of three stages of tunable generalized Mach–Zehnder interferometers. All 24 routing states for nonblocking switching are characterized. The device’s footprint is 4.6 mm × 1.0 mm. Measurements show that the worst cross talk of all switching states is ?7.2 dB. The on-chip insertion loss is in the range of 3.7–13.1 dB. The average TO switching power consumption is 104.8 mW.