Using a spherical bipolar coordinate system, an exact analytical solution is obtained for the thermocapillary motion of a spherical droplet settling in the presence of a plane interface formed by the contact of two immiscible viscous fluids. A uniform temperature gradient is applied to the system in a direction perpendicular to the plane interface. The appropriate field equations of energy and momentum are solved in the quasi-steady limit under the conditions of small Péclet and Reynolds numbers. In this study, we also assumed that the capillary numbers at the interface of the droplet or at the separating plane surface are respectively small to maintain the spherical shape of the droplet and that the flat shape of the separating surface is permanent during the motion. The novelty of these three fluid phases problem is to find the thermocapillary velocity of the droplet and study the effect of the separating surface and the thermal conductivities of the system on the thermocapillary velocity. It is found that the interaction between the droplet and the separating surface can be strong in the case when the droplet is almost in contact with the separating surface. Several limiting cases are discussed and compared with the relevant cases available in the literature. The motivation of the study is its potential applications in many branches of chemical, biomedical, and environmental engineering, such as the locomotion of microswimmers near an interface.