The invention relates to a passive mechanical athermalization device, comprising: a barrel (1) that is made of a first material having a first thermal expansion coefficient (11) and that has a longitudinal axis (AA), said barrel comprising at least one first portion (11) and at least one second portion (12), the device being characterized in that it comprises: at least three beams (7) made of the first material, each of the beams (7) circumferentially connecting the first portion (11) and the second portion (12) relative to the longitudinal axis (AA); at least three bars (8) made of a second material having a second thermal expansion coefficient (12) that is different from the first thermal expansion coefficient, circumferentially distributed around the barrel (1) relative to the longitudinal axis (AA), each bar (8) axially connecting the first portion (11) and the second portion (12) relative to the longitudinal axis (AA) such that the thermal expansion of the barrel (1) results in a deformation of the beams (7) and of the bars (8) along the longitudinal axis (AA). The invention also relates to an optical system comprising such a passive athermalization device.