An improved sinker for circular hosiery knitting machines, with high resistance to the danger of damage causable by contiguous needles. The sinker (1) comprises a laminar body (2) which can be arranged on a vertical plane that passes through the axis (3a) of the needle cylinder (3) of a circular hosiery knitting machine between two contiguous needles (7) of the machine which are accommodated in respective axial slots (6) defined on the lateral surface of the needle cylinder (3). Each one of the needles (7) is movable with a reciprocating motion along the corresponding axial slot (6) with respect to the needle cylinder (3). The laminar body (2) has, on its front side, intended to be directed toward the axis (3 a) of the needle cylinder (3), a nose (8) which defines, with its upper side, the knockover plane (9) of the sinker (1). The sides (30) of the laminar body (2), in their regions intended to be superimposed laterally on the paths (12) of the contiguous needles (7) as a result of the reciprocating motion of the needles (7) along the corresponding axial slots (6) of the needle cylinder (3) and of the reciprocating motion of the sinker (1), on its plane of arrangement, toward and away from the axis (3 a) of the needle cylinder (3), have a continuous flat surface.