A ski boot in plastic material includes a shell adapted to receive the foot, a front spoiler articulated on the shell for pivoting frontwards thereof and adapted to grip the bottom part of the leg, a buckle system for tightening the front spoiler on the bottom part of the leg, and a damper joined to the shell and to the front spoiler. A flexible arciform plate is further provided which adopts the general shape of the instep, an upper end of the plate being joined to the front spoiler, and a lower end of the plate being joined to the shell, the upper end of said arciform bending plate cooperating with a compressible elastic return element during leaning forward movements.