The sirtuins are NAD+-dependent lysine deacylases, comprising seven isoforms (SIRT1-7) in humans, which are involved in the regulation of a plethora of biological processes, including gene expression and metabolism. The sirtuins share a common hydrolytic mechanism but display preferences for different epsilon-N-acyllysine substrates. SIRT7 deacetylates targets in nuclei and nucleoli but remains one of the lesser studied of the seven isoforms, in part due to a lack of chemical tools to specifically probe SIRT7 activity. Here we expressed SIRT7 and, using small-angle X-ray scattering, reveal SIRT7 to be a monomeric enzyme with a low degree of globular flexibility in solution. We developed a fluorogenic assay for investigation of the substrate preferences of SIRT7 and to evaluate compounds that modulate its activity. We report several mechanism-based SIRT7 inhibitors as well as de novo cyclic peptide inhibitors selected from mRNA-display library screening that exhibit selectivity for SIRT7 over other sirtuin isoforms, stabilize SIRT7 in cells, and cause an increase in the acetylation of H3 K18.|Broad screening of the substrate specificity of SIRT7 revealed a strong preference for the cleavage of long-chain acyllysine modifications. The activity was substantially increased in the presence of oligonucleotides or nucleosome particles. Finally, random nonstandard peptide integrated discovery (RaPID) delivered a cyclopeptide with potency in the sub-micromolar range and selectivity for SIRT7, which binds and stabilizes SIRT7 in cells.**+image