Dipeptidyl peptidase 9 (DPP9) is a proline-selective serine protease that plays a key role in NLRP1- and CARD8-mediated inflammatory cell death (pyroptosis). No selective inhibitors have hitherto been reported for the enzyme: all published molecules have grossly comparable affinities for DPP8 and 9 because of the highly similar architecture of these enzymes' active sites. Selective DPP9 inhibitors would be highly instrumental to address unanswered research questions on the enzyme's role in pyroptosis, and they could also be investigated as therapeutics for acute myeloid leukemias. Compounds presented in this manuscript (42 and 47) combine low nanomolar DPP9 affinities with unprecedented DPP9-to-DPP8 selectivity indices up to 175 and selectivity indices >1000 toward all other proline-selective proteases. To rationalize experimentally obtained data, a molecular dynamics study was performed. We also provide in vivo pharmacokinetics data for compound 42.