The present invention discloses a temperature compensation device for optical instruments which comprises a lens assembly, motor means for rotating the lens assembly about the optical axis thereof, kinematic means for producing rectilinear motion of the lens assembly in response to the rotary motion of the lens assembly, and means for determining the linear displacement and therefore the actual linear position of the lens assembly. Additionally, the device comprises thermometric means, e.g. a temperature detector, which measures the temperature of the environment and transmits the temperature to a processor which controls the actual linear position of the lens assembly through an electronic circuit. The relationship between the temperature detector reading and the corresponding desired linear position is defined by memorized means, preferably by a LUT (look-up table) or, e.g., through optimization on a sensor (ccd, IR, etc.) output (picture). At all times, the desired linear position, corresponding to the temperature detector reading, is compared to the actual linear position, and, if there is a difference, the aforesaid kinematic means is actuated by appropriate means to displace the lens assembly to the desired linear position.