Chaperones are abundant cellular proteins that promote the folding and function of their substrate proteins (clients). In vivo, chaperones also associate with a large and diverse set of cofactors (cochaperones) that regulate their specificity and function. However, how these cochaperones regulate protein folding and whether they have chaperone-independent biological functions is largely unknown. We combined mass spectrometry and quantitative high-throughput LUMIER assays to systematically characterize the chaperone-cochaperone-client interaction network in human cells. We uncover hundreds of chaperone clients, delineate their participation in specific cochaperone complexes, and establish a surprisingly distinct network of protein-protein interactions for cochaperones. As a salient example of the power of such analysis, we establish that NUDC family cochaperones specifically associate with structurally related but evolutionarily distinct β-propeller folds. We provide a framework for deciphering the proteostasis network and its regulation in development and disease and expand the use of chaperones as sensors for drug-target engagement. Display omitted •Client interactions for >60 chaperones and cochaperones mapped by AP-MS and LUMIER•Characterization of cellular roles of cochaperones and their client specificities•NUDC family cochaperones associate with β-propeller domains (Kelch, WD40, and RCC1)•Cochaperones may promote the evolutionary diversification of client folds Mass spectrometry and quantitative LUMIER assays map the proteostasis network in human cells, revealing hundreds of new client proteins, their integration into the network, and the client specificity of most cochaperones.