The crystal structure, magnetic and magnetothermal properties of (TmxPr1-x)2Fe16.5Nb0.5 alloys were studied. Alloys with x = 0–0.4 crystallize into a rhombohedral structure of the Th2Zn17-type, and alloys with x = 0.6–1 crystallize into a structure of the LuFe9.5-type, which is a disordered variant of the hexagonal structure of the Th2Ni17-type. The concentration range of single-phase hexagonal compositions turned out to be wide in the system (TmxPr1-x)2Fe16.5Nb0.5 and equal to x = 0.6–1. The rhombohedral and hexagonal phases coexist only in one alloy x = 0.5, and the Curie temperatures of these phases differ unexpectedly greatly by 14.1 K. The lattice parameters, saturation magnetization, and magnetocaloric effect -ΔSM increase in the system as the Pr content increases. The refrigerant capacity RC = 78.7 J/kg in a field of 15 kOe for an alloy with x = 0.5, in which two peaks -ΔSM(T) overlap, is the maximum in the system. The Curie temperature in the system increases nonmonotonically from 312.3 K for Pr2Fe16.5Nb0.5 to 346.2 K for Tm2Fe16.5Nb0.5 and deviates maximum from the linear dependence for x = 0.8. Apparently, this deviation of TC(x) is due to the influence of the internal pressure determined for the (TmxPr1-x)2Fe16.5Nb0.5 alloys by X-ray diffraction analysis.