Exocytosis involves membrane fusion between granules and the plasma membrane. Nitric oxide (NO) inhibits exocytosis by chemically modifying N-ethylmaleimide-sensitive factor (NSF), a key component of the exocytic machinery. However, cells recover the ability to release messenger molecules within hours of exposure to NO through unknown mechanisms. We now identify thioredoxin (TRX1) as a denitrosylase that reverses NO inhibition of exocytosis. Endogenously synthesized NO increases S-nitrosylated NSF levels, but S-nitrosylated NSF levels decrease within 3 h after exposure to NO. We found that NO increases the interaction between TRX1 and NSF, and endogenous TRX1 removes NO from S-nitrosylated NSF. Knockdown of TRX1 increases the level of S-nitrosylated NSF, prolongs the inhibition of exocytosis, and suppresses leukocyte adhesion. Taken together, these data show that TRX1 promotes exocytosis by denitrosylating NSF. Our findings suggest that TRX1 might regulate exocytosis in a variety of physiological settings, such as vascular inflammation, thrombosis, and insulin release.