Problem: To provide a side airbag device that can improve restraint performance by efficiently suppressing oscillation of an airbag cushion. Resolution Means: A side airbag device 100 is provided with: a vertically long rear chamber 116, a front chamber 118 that is inflated and deployed forward and upward from the rear chamber 116, an inflator 108 contained within a rear part of the rear chamber 116, and a cloth-like first tether 112 that is provided outside of the rear chamber 116, has a front end 122 connected to a front edge portion 124 of the rear chamber 116, and has a rear end 126 connected to the rear part of the rear chamber 116, and which is tensioned when the rear chamber 116 is inflated and deployed. The rear chamber 116 has a smaller capacity than the front chamber 118, the first tether 112 is longer in the vertical direction than in the front-back direction, and the front end 122 of the first tether 112 is connected to a range that includes at least a lower end 146 of the front edge portion 124 of the rear chamber 116.