In this paper, we report a series of novel mixed‐metal‐substituted organic‐inorganic hybrid polyoxometalates (POMs) H3[(CH3)4N]14[NaLn(PW11O39)3]·18H2O (1‐Ln, Ln = Nd, Sm, Eu) via one‐pot solvothermal synthesis methods, which present the first series of polyoxometalate trimers composed of both alkali‐metal‐ and lanthanide‐substituted keggin‐type POMs. The trimeric heterometallic polyoxoanions in 1‐Ln show an interesting linear arrangement and can be further packed together to generate a packing structure with cations [(CH3)4N]+ and H3O+ filling the gaps between polyoxoanions. Compounds 1‐Ln are characterized by single‐crystal X‐ray diffraction analysis, TGA, IR and PXRD. Additionally, the luminescence and proton conductive properties of 1‐Ln are also characterized.