Device for moulding thermoplastic containers by the blow moulding of preforms, comprising a mould (1) and a blowing assembly (2) with a nozzle (16) that can move axially between a raised position and a blowing position in fluid connection with a preform supported in the cavity of the mould. The half-moulds (3a, 3b) of the mould bear respective parts (Aa, Ab) together forming a male element (A) bordered by a wall (12) inclined towards the nozzle. The blowing assembly bears a female element (B) formed by a clamp (26) having an internal wall (30) complementary to the wall of the protuberance. In the blowing position, the cooperation between the male and female elements (A, B) locks the upper part of the closed mould.