A bathing auxiliary chair suitable for bathtubs of different sizes generally includes a seating unit and a supporting unit. The seating unit, which allows a user to sit thereon while bathing, is supported by the supporting unit and can be moved between a first position and a second position. The seating unit is provided with a front guide shell and two rear guide shells. In use, when the seating unit is located at the first position or the second position, one of the rear guide shells can abut against a fastener located at a rear tube of the supporting unit, and the front guide shell can be coupled to a front tube of the supporting unit through a button of a snap mechanism, so that the seating unit can be fixed at the position reliably, and the bathing auxiliary chair can be used safely.
