<b><i>Background:</i></b> Somatic mutations in the ubiquitin-specific protease 8 (USP8) gene have recently been shown to occur in ACTH-secreting pituitary adenomas, thus calling attention to the ubiquitin system in corticotrope adenomas. <b><i>Objectives:</i></b> Assess the consequences of <i>USP8</i> mutations and establish the role of ubiquitin on ACTH turnover in human ACTH-secreting pituitary adenomas. <b><i>Methods:</i></b> <i>USP8</i> mutation status was established in 126 ACTH-secreting adenomas. Differences in ACTH secretion and <i>POMC</i> expression from adenoma primary cultures and in microarray gene expression profiles from archival specimens were sought according to <i>USP8</i> sequence. Ubiquitin/ACTH coimmunoprecipitation and incubation with MG132, a proteasome inhibitor, were performed in order to establish whether ubiquitin plays a role in POMC/ACTH degradation in corticotrope adenomas. <b><i>Results:</i></b> <i>USP8</i> mutations were identified in 29 adenomas (23%). Adenomas presenting <i>USP8</i> mutations secreted greater amounts of ACTH and expressed <i>POMC</i> at higher levels compared to <i>USP</i> wild-type specimens. <i>USP8</i> mutant adenomas were also more sensitive to modulation by CRH and dexamethasone in vitro. At microarray analysis, genes associated with endosomal protein degradation and membrane components were downregulated in <i>USP8</i> mutant adenomas as were <i>AVPR1B</i>, <i>IL11RA</i>, and<i> PITX2</i>. Inhibition of the ubiquitin-proteasome pathway increased ACTH secretion and POMC itself proved a target of ubiquitylation, independently of <i>USP8</i> sequence status. <b><i>Conclusions:</i></b> Our study has shown that <i>USP8</i> mutant ACTH-secreting adenomas present a more “typical” corticotrope phenotype and reduced expression of several genes associated with protein degradation. Further, ubiquitylation is directly involved in intracellular ACTH turnover, suggesting that the ubiquitin-proteasome system may represent a target for treatment of human ACTH-secreting adenomas.