Y-family DNA polymerases participate in replication stress and DNA damage tolerance mechanisms. The properties that allow these enzymes to copy past bulky adducts or distorted template DNA can result in a greater propensity for them to make mistakes. Of the four human Y-family members, human DNA polymerase iota (hpol t) is the most error-prone. In the current study, we elucidate the molecular basis for improving the fidelity of hpol t through use of the fixed-conformation nucleotide North-methanocarba-2'-deoxyadenosine triphosphate (N-MC-dATP). Three crystal structures were solved of hpol t in complex with DNA containing a template 2'-deoxythymidine (dT) paired with an incoming dNTP or modified nucleotide triphosphate. The ternary complex of hpol t inserting N-MC-dATP opposite dT reveals that the adenine ring is stabilized in the anti orientation about the pseudo-glycosyl torsion angle, which mimics precisely the mutagenic arrangement of dGTP:dT normally preferred by hpol t. The stabilized anti conformation occurs without notable contacts from the protein but likely results from constraints imposed by the bicyclo[3.1.0]hexane scaffold of the modified nucleotide. Unmodified dATP and South-MC-dATP each adopt syn glycosyl orientations to form Hoogsteen base pairs with dT. The Hoogsteen orientation exhibits weaker base-stacking interactions and is less catalytically favorable than anti N-MC-dATP. Thus, N-MC-dATP corrects the error-prone nature of hpol t by preventing the Hoogsteen base-pairing mode normally observed for hpol t-catalyzed insertion of dATP opposite dT. These results provide a previously unrecognized means of altering the efficiency and the fidelity of a human translesion DNA polymerase.