An extremely long ribbon-shaped polyoxometalate (POM) octamer, {Eu16Co7Se16W128O448-(CIT)(10)(HCIT)(2)(NO3)(4)(OH)(4)(H2O)(52)} (H4CIT = citric acid), with a maximum length of ca. 6 nm has been obtained, which is the longest molecular POM reported to date and the first Dawson-based 3d-4f-containing selenotungstate(IV). The nanoscale octamer is built from 8 new Dawson-type Eu2W2-substituted selenotungstates {(Eu2W2O4)Se2W14O52} bridged by 12 H4CIT ligands and 3 Co2+ ions, giving rise to a rare giant inorganic-organic hybrid molecular POM with more than 150 metal centers (7 Co-II, 16 Eu-III, 16 Se-IV, and 128 W-VI). Especially, with 128 W centers and more than 800 non-H atoms, the POM octamer also represents the largest known 3d-4f-containing polyoxotungstate.