Numerous experimental data are available on contact angles. However, the majority of them concern sessile droplets, although bubbles are more interesting from the point of view of colloid science, because a sessile bubble is distinguished by the existence of an equilibrium thin liquid film at a gas–solid interface. This paper presents the totality of modern thermodynamic equations necessary for describing the contact angle of a bubble of an arbitrary size. However, in view of their complexity, specific relationships are derived and analyzed for small although macroscopic bubbles. The dependences of a bubble contact angle on the temperature, pressure in a liquid, composition of a liquid phase, and roughness of a solid surface are considered. In the case of a pure liquid, when a gaseous phase consists of its vapor, the obtained relations are of significance for the cavitation theory. The study as a whole has a framework character, and so only the general basic results are presented. Their detailing is possible in combination with experimental data.