Porcupine is a member of the membrane-bound O-acyltransferase family of proteins. It catalyzes the palmitoylation of Wnt proteins, a process required for their secretion and activity. We recently disclosed a class of small molecules (IWPs) as the first reported Porcn inhibitors. We now describe the structure–activity relationship studies and the identification of subnanomolar inhibitors. We also report herein the effects of IWPs on Wnt-dependent developmental processes, including zebrafish posterior axis formation and kidney tubule formation.