We report the discovery of ~10 mHz quasi-periodic oscillations (QPOs) in a 3.5 ks RXTE observation of the black hole candidate IGR J17091-3624 (ATELs #3144, #3159, #3167, #3203) on March 14th 2011 (MJD 55634.66). Only PCU2 was active, with an average 2-60 keV background-corrected count rate of 110 counts/s. The QPO is visible by eye in the light curve during the first ~1000 seconds and the last ~1500 seconds, with an average 2-60 keV fractional rms amplitude of 5.3+-0.4% and a mean frequency of ~10 mHz.