Disclosed is a seat recliner for a vehicle, the seat recliner including: a first flange provided with a plurality of guide parts being spaced apart from each other, with guide spaces defined between the guide parts; a plurality of locking gears disposed in the guide spaces of the first flange respectively, configured to slide forward and backward along the guide parts in a radial direction, and provided with external teeth at an outer end of each of the locking gears; and a second flange coupled with the first flange to be rotatable relative thereto, formed in a shape covering the guide parts and the locking gears, and configured to be engaged with the external teeth of the locking gears.