At nuclear pores, a short membrane region connecting the inner and the outer nuclear membrane serves as a platform where nucleoporins with one or more transmembrane domains promote anchoring of the nuclear pore complex to the nuclear envelope. In mammalian cells, three transmembrane nucleoporins, Nup210, POM121 and NDC1, are inserted at this position. Here we characterize TMEM209, which had initially been identified as a protein concentrated at the nuclear envelope, as a fourth transmembrane nucleoporin. Proximity labeling showed that TMEM209 occurs close to proteins of the inner nuclear membrane and to other nucleoporins. TMEM209 strongly targeted to the nuclear pore complex in immunolocalization and biochemically interacted with Nup210 via a region containing its two transmembrane domains. The depletion of TMEM209 resulted in reduced cell growth with delayed entry into S-, G2 and M-phases. Ectopic overexpression of TMEM209, on the other hand, specifically dissociated Nup210 from the nuclear envelope. In summary, we establish TMEM209 as a novel nucleoporin that, together with its interaction partner protein Nup210, is involved in cell cycle progression and cell proliferation.