Five novel lanthanoid-containing silicotungstates with polymeric crystal structures [Ln2(H2O)7Si2W18O66]n10n– [Ln = GdIII (Gd-1 and Gd-2), TbIII, HoIII] and [Dy2(H2O)6.5(C2H4O2)0.5Si2W18O66]n10n– were obtained from the one-step reaction of Na10[SiW9O34]·nH2O with Ln(NO3)3·nH2O in a sodium acetate buffer. The compounds were characterized by single-crystal X-ray diffraction and a wide range of analytical methods, including FT-IR, UV/vis, and photoluminescence spectroscopy as well as electrochemistry and thermogravimetric analysis. This new polyoxotungstate series is the first example of lanthanoids embedded in the open Wells–Dawson silicotungstate anion [α-Si2W18O66]16–. The lanthanoid-containing Wells–Dawson-type polyoxoanions [Ln2(H2O)7Si2W18O66]10– [Ln = GdIII (Gd-1 and Gd-2), TbIII, HoIII] and [Dy2(H2O)6.5(C2H4O2)0.5Si2W18O66]10– are linked by Ln3+ cations to form 3D architectures for Gd-1 or 2D frameworks for the isostructural compounds Tb-2, Dy-2, Ho-2, and Gd-2. The structure-directing influence of the lanthanoid cation on the local structure of the dimeric building blocks and on the crystal packing motifs is investigated in detail. The photoluminescence properties of Tb-2 and Dy-2 were investigated at room temperature, and Ho-2 exhibits an interesting photochromic behavior. The magnetic susceptibility of Gd-1 and Gd-2 was studied in the temperature range between 2 and 300 K for its effective magnetic moment.