Circadian clock is an internal system synchronized by external stimuli such as light and temperature and influences various physiological and developmental processes in living organisms. In Arabidopsis as a model plant, transcriptional, translational and post-translational processes are interlocked as by feedback loops between morning- and evening-phased genes. In a post-translational loop, plant-specific single-gene encoded GIGANTEA (GI) stabilize the F-box protein ZEITLUPE (ZTL) leading targeted-proteasomal degradation of TIMING OF CAB EXPRESSION 1 (TOC1) and PSEUDO-RESPONSE REGULATOR 5 (PRR5). Here, we showed that a novel biochemical function of GI as a chaperone and/or co-chaperone of Heat-Shock Protein 90 (HSP90). GI itself prevents ZTL degradation as a chaperone and facilitates ZTL maturation together with HSP90/HSP70 enhancing ZTL activity in vitro and in planta. GI has been known to be involved in a wide range of physiology and development as well as abiotic stress responses in plants, and it could interact with diverse client proteins for protein maturation. Our results provide evidence that GI helps proteostasis of ZTL by acting as a chaperone and a co-chaperone of HSP90 for proper running of circadian clock in Arabidopsis.