Embryonic stem cells can replicate continuously in the absence of senescence and, therefore, are immortal in culture(1,2). Although genome stability is essential for the survival of stem cells, proteome stability may have an equally important role in stem-cell identity and function. Furthermore, with the asymmetric divisions invoked by stem cells, the passage of damaged proteins to daughter cells could potentially destroy the resulting lineage of cells. Therefore, a firm understanding of how stem cells maintain their proteome is of central importance. Here we show that human embryonic stem cells (hESCs) exhibit high proteasome activity that is correlated with increased levels of the 19S proteasome subunit PSMD11 (known as RPN-6 in Caenorhabditis elegans)(3-5) and a corresponding increased assembly of the 26S/30S proteasome. Ectopic expression of PSMD11 is sufficient to increase proteasome assembly and activity. FOXO4, an insulin/insulin-like growth factor-I (IGF-I) responsive transcription factor associated with long lifespan in invertebrates(6,7), regulates proteasome activity by modulating the expression of PSMD11 in hESCs. Proteasome inhibition in hESCs affects the expression of pluripotency markers and the levels of specific markers of the distinct germ layers. Our results suggest a new regulation of proteostasis in hESCs that links longevity and stress resistance in invertebrates to hESC function and identity.