Notum is a carboxylesterase that suppresses Wnt signaling through deacylation of an essential palmitoleate group on Wnt proteins. There is a growing understanding of the role Notum plays in human disease such as colorectal cancer and Alzheimer’s disease supporting the need to discover improved inhibitors, especially for use in models of neurodegeneration. Here, we describe the discovery and profile of 8I (ARUK3001185) as a potent, selective and brain pentrant inhibitor of Notum activity suitable for oral dosing in rodent models of disease. Crystallographic fragment screening of the Diamond-SGC Poised Library for binding to Notum, supported by a biochemical enzyme assay to rank inhibition activity, identifed a pair of outstanding hits. Fragment development of these delivered 8I that restored Wnt signaling in the presence of Notum in a cell-based reporter assay. Assessment in proteomic and pharmacology screens showed 8I to be selective against serine hydrolases, kinases and drug targets.