Nitronium tetrafluoroborate is an inorganic compound with formula NO₂BF₄. It is a salt of nitronium cation and tetrafluoroborate anion. It is a colorless crystalline solid, which reacts with water to form the corrosive acids HF and HNO₃. As such, it must be handled under water-free conditions. It is sparsely soluble in many organic solvents.