Solvothermal reactions of elements Ge and Se with Ln₂O₃ in a pentadentate polyamine, tetraethylenepentamine (tepa), produced novel neutral lanthanide–selenidogermanate polymers [{Ln(tepa)(μ-OH)}₂(μ-Ge₂Se₈)]_n (Ln = Eu 1, Gd 2, Dy 3). The reaction with Dy₂O₃ in ethylenediamine (en) afforded an ionic selenidogermanate [{Dy(en)₃}₂(μ-OH)₂]Ge₂Se₆ (4). In compounds 1–3, the Ln³⁺ ions are coordinated by a tepa and two OH⁻ ligands to form binuclear [{Ln(tepa)}₂(μ-OH)₂]⁴⁺ fragments. Two GeSe₄ tetrahedra are linked through two Se–Se bonds to form a novel [Ge₂Se₈]⁴⁻ unit containing a six-membered Ge₂Se₄ ring in the chair conformation. The [Ge₂Se₈]⁴⁻ unit acts as a bridging ligand via the trans terminal Se atoms to interlink the [{Ln(tepa)}₂(μ-OH)₂]⁴⁺ fragments into one-dimensional polymers [{Ln(tepa)(μ-OH)}₂(μ-Ge₂Se₈)]_n. Compounds 1–3 are the first examples of solvothermally prepared lanthanide complexes with a selenidogermanate anion as a ligand. The [Ge₂Se₆]⁴⁻ anion in 4 is composed of two GeSe₄ tetrahedra sharing a common edge, and is charge compensated by a [{Dy(en)₅}₂(μ-OH)₂]⁴⁺ complex cation. The formation of the [Ge₂Se₈]⁴⁻ and [Ge₂Se₆]⁴⁻ anions and their behavior towards lanthanide ions in 1–4 show the significant influence of ethylene polyamines on the solvothermal synthesis of Ln selenidogermanates.