The polytypism of layered crystals of thallium gallium diselenide TlGaSe2 has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe2 polytype is an improper ferroelectric phase transition occurring at a temperature Tc ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe2 polytype is a proper ferroelectric phase transition occurring at a higher temperature Tc ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe2 crystals.