ABSTRACT A highly curved 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 site. Here, we characterize TMEM209, which had initially been identified as a protein concentrated at the nuclear envelope, as a fourth transmembrane nucleoporin. Proximity labeling revealed that TMEM209 is present close to proteins of the inner nuclear membrane and to other nucleoporins. TMEM209 localized to the nuclear pore complex in immunofluorescence microscopy and biochemically interacted with Nup210 via a region containing its two transmembrane domains. TMEM209 depletion impaired cell growth and delayed entry into S, G2 and M phases of the cell cycle. Conversely, its overexpression specifically dissociated Nup210 from the nuclear envelope. Together, these findings establish TMEM209 as a novel transmembrane nucleoporin that cooperates with Nup210 in cell cycle progression and cell proliferation.