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Self‐focusing of an intense optical beam in a plasma is studied, including the nonlinear effects of both the relativistic electron mass and the ponderomotive potential due to the electromagnetic wave. An exact steady asymptotic solution of beam propagation in a localized solitary wave form is obtained in slab geometry. Amplitude‐width scaling relations are obtained, which imply that the width is limited to be less than square root of three of the collisionless skin depth. In the nonrelativistic limit, keeping only the relativistic mass effects, our solution reduces to the solution obtained by Schmidt and Horton. The asymptotic nature of the solitary wave is tested using a recently developed numerical particle simulation code.