Heat shock protein 90 (Hsp90) is a molecular chaperone regulating the activity of diverse client proteins together with a plethora of different co-chaperones. Whether these functionally cooperate has remained enigmatic. We analyze all double mutants of 11 Saccharomyces cerevisiae Hsp90 co-chaperones in vivo concerning effects on cell physiology and the activation of specific client proteins. We find that client activation is supported by a genetic network with weak epistasis between most co-chaperones and a few modules with strong genetic interactions. These include an epistatic module regulating protein translation and dedicated epistatic networks for specific clients. For kinases, the bridging of Hsp70 and Hsp90 by Sti1/Hop is essential for activation, whereas for steroid hormone receptors, an epistatic module regulating their dwell time on Hsp90 is crucial, highlighting the specific needs of different clients. Thus, the Hsp90 system is characterized by plastic co-chaperone networks fine-tuning the conformational processing in a client-specific manner. Display omitted •Genetic interactions between Hsp90 co-chaperones are dynamic and client specific•Sti1, Cpr7, and Cns1 form an epistatic module maintaining eEF2 integrity•Sti1/Hsp70 and Cdc37 act on two parallel pathways for kinase maturation•Aha1 and Hch1 are dwell-time regulators for Hsp90 client complexes Biebl et al. show that Hsp90 co-chaperones form client-specific genetic networks for client maturation. Tightly interacting core genetic modules of co-chaperones specific for a client are embedded in a network of looser genetic interactions. Distinct epistatic Hsp90 co-chaperone modules regulate kinase activation, steroid hormone receptor maturation, and protein biosynthesis.