rTMS is a treatment that involves stimulating a certain area of the brain with magnetic field pulses. Over time, the magnetic field pulses can gradually change the activity level of the stimulated brain region and help symptoms of bipolar depression. The device used in this study has been approved by Health Canada for therapeutic use since 2002. Participants will complete a screen visit to determine eligibility based on the inclusion/exclusion criteria. If the participants are not eligible, no further study procedures will be conducted. Eligible subjects will be randomized to receive either active iTBS-rTMS or sham rTMS treatment (scalp stimulation with no magnetic pulse) daily for four weeks (20 sessions) to the left dorsolateral prefrontal cortex (DLPFC). All participants will complete a MRI (to target the left DLPFC region of the brain and functional activity), EEG & fNIRS, lab work, and neurocognitive testing prior to the commencement and post rTMS treatment. Efficacy, safety and tolerability will be evaluated at screen visit, during daily rTMS treatments, clinic visits and post rTMS treatment. All participants will have a phone interview two weeks post rTMS treatment.