The 14-3-3 proteins are highly conserved molecules that function as intracellular adaptors in a variety of biological processes, such as signal transduction, cell cycle control, and apoptosis. Here, we show that a 14-3-3 protein is a heat-shock protein (Hsp) that protects cells against physiological stress as its new cellular function. We have observed that, in Drosophila cells, the 14-3-3zeta is up-regulated under heat stress conditions, a process mediated by a heat shock transcription factor. As the biological action linked to heat stress, 14-3-3zeta interacted with apocytochrome c, a mitochondrial precursor protein of cytochrome c, in heat-treated cells, and the suppression of 14-3-3zeta expression by RNA interference resulted in the formation of significant amounts of aggregated apocytochrome c in the cytosol. The aggregated apocytochrome c was converted to a soluble form by the addition of 14-3-3zeta protein and ATP in vitro. 14-3-3zeta also resolubilized heat-aggregated citrate synthase and facilitated its reactivation in cooperation with Hsp70/Hsp40 in vitro. Our observations provide the first direct evidence that a 14-3-3 protein functions as a stress-induced molecular chaperone that dissolves and renaturalizes thermal-aggregated proteins.