Proteasome‐mediated proteolysis is critical for regulation of vast majority of cellular processes. In addition to their well‐documented functions in the nucleus and cytoplasm proteasomes have also been found in extracellular space. The origin and functions of these proteasomes, dubbed as circulating/plasmatic or extracellular proteasomes, are unclear. To gain insights into the molecular and functional differences between extracellular (EPs) and cellular proteasomes (CPs) we compared their subunit composition using iTRAQ‐based quantitative proteomics (iTRAQ LC/MS‐MS). Our analysis of purified from K562 cells or conditioned medium intact proteasome complexes led to an identification and quantification of 114 proteins, out of which 19 were 26S proteasome proteins (all subunits of the 20S proteasome and a small number of the 19S regulatory particle proteins), and 3 belonged to the ubiquitin system. Sixty‐two of proteasome interacting proteins (PIPs) were differentially represented in CP versus EP, with folds difference ranging from 1.5 to 4.8. The bioinformatics analysis revealed that functionally most of EP‐PIPs were associated with protein biosynthesis and, unlike CP‐PIPs, were under represented by chaperon/ATP‐binding proteins. Identities of some of the proteasome proteins and PIPs were verified by Western blotting. Importantly, we uncovered that the stoichiometry of the 20S versus 19S complexes in the extracellular proteasomes was different compared to the one calculated for the intracellular proteasomes. Specifically, the EP prep contained only three 19S subunits versus at least 18 in the CP one, suggesting that the extracellular proteasomes are deficient in 19S complexes, which may imply that they have special biological functions. J. Cell. Physiol. 232: 842–851, 2017. © 2016 Wiley Periodicals, Inc. To gain insights into the molecular and functional differences between extracellular (EPs) and cellular proteasomes (CPs) we compared their subunit composition using iTRAQ‐based quantitative proteomics (iTRAQ LC/MS‐MS). Importantly, we uncovered that the stoichiometry of the 20S versus 19S complexes in the extracellular proteasomes was different compared to the one calculated for the intracellular proteasomes, which implies that the two species of proteasomes may have different biological functions.