Vertebrates have evolved an understudied protein termed CDADC1 (NYD-SP15) that contains an inactive N-terminal and active C-terminal DCTD-like domain. Here, we show that human CDADC1 is a (d)CTP-specific deaminase, with a roughly 2-fold in vitro preference for dCTP over CTP. We determined high-resolution cryo-EM structures of CDADC1 in the absence of substrate and in complex with dCTP and 5-methyl-dCTP. The structures show that CDADC1 forms trimers and dimers of trimers in solution. The (d)CTP substrate is selected by a narrow pocket for the cytosine base and multiple lysine and arginine contacts to the triphosphate. Substrate binding promotes the association of trimers into hexamers and the transition of the hexamers from a loose to a tighter arrangement. Genetic experiments in mice show that loss of Cdadc1 is surprisingly well tolerated, even in the absence of the dCMP deaminase Dctd that is considered as the main source of dUMP, the precursor of dTTP.