The TMEM175 family constitutes recently discovered K+ channels that lack signatures for a P-loop selectivity filter, a hallmark of all known K+ channels. This raises the question how selectivity in TMEM175 channels is achieved. Here we report the X-ray structure of a bacterial TMEM175 family member in complex with a novel chaperone built of a nanobody fusion-protein. The structure of the channel in a non-conductive conformation was solved at 2.4 Å and revealed bound K+ ions along the channel pore. A hydrated K+ ion at the extracellular pore entrance that could be substituted with Cs+ and Rb+ is coordinated by backbone-oxygens forming a cation-selective filter at the tip of the pore-lining helices. Another K+ ion within the pore indicates the passage of dehydrated ions. Unexpectedly, a highly conserved threonine residue deeper in the pore conveys the K+ selectivity. The position of this threonine in the non-conductive state suggests major conformational rearrangements of the pore-lining helices for channel opening, possibly involving iris-like motions.