Thermoplastic container (1) which after filling and sealing is able to withstand without any substantial deformation to an internal pressure lower than atmospheric pressure, comprising a body (2) with substantially cylindrical upper (3) and lower (4) parts and an intermediate part (5) having a curvilinear triangular cross section and comprising three main faces (6a, 6b, 6c) joined by three edges (7a, 7b, 7c); these edges are rounded, having continuous curvature, joined tangentially to the main faces and have radii of curvature (ra, rb, rc) comprised between approximately 5% and 35% of the largest dimension (D) of the cross section of the lower part (3); the main faces have radii of curvature (Ra, Rb, Rc) greater than the largest dimension (D) of the cross section of the lower part (3).