Disclosed is a side airbag for vehicles including a first cushion coupled to one side of a seat, the first cushion extending from the side surface of the seat in a forward-rearward direction to cover a passenger at the side of the passenger when deployed, an inflator fixed to the seat, the inflator being configured to supply gas to the first cushion, a second cushion connected to the first cushion, the second cushion being bent from the first cushion toward the passenger and extending in a lateral direction, the second cushion being deployed by the gas supplied from the first cushion, and a support tether having one end fixedly coupled to the second cushion and the other end fixedly coupled to the seat, wherein the support tether is bent so as to be slidably coupled to the first cushion between one end and the other end thereof.