Fibroblast growth factor 21 (FGF21) is an endocrine hormone with broad metabolic actions at supraphysiological concentrations, but unclear physiological function, with recent data suggesting a link to endoplasmic reticulum (ER)-stress. ER-stress activates the unfolded protein response (UPR), a cellular repair mechanism maintaining cellular homeostasis during protein folding stress. Using APEX2 proximity labelling, we assessed the intracellular action of FGF21 at its receptor β-klotho (KLB), and discovered associations to protein folding in the ER, ER-stress, and H2S production. We found that FGF21 enhances the UPR, and that this action is blunted by either genetic or pharmacological inhibition of sulfide signaling, and phenocopied by an H2S-donor in vivo . Strikingly, we found that even physiological levels of FGF21 KLB-dependently modulate the UPR via increased hepatic H2S production. Our data hence identify a novel physiological role of FGF21 and further indicate that sulfide signaling is a core second messenger of the UPR.