Boron trifluoride etherate, strictly boron trifluoride diethyl etherate, or boron trifluoride–ether complex, is the chemical compound with the formula BF₃O(C₂H₅)₂, often abbreviated BF₃OEt₂. It is a colorless liquid, although older samples can appear brown. With the flash point between 37.8 and 93.3 °C, BF3OEt₂ was chemically active. BF₃OEt₂ could undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. It reacts exothermically with water to form extremely flammable diethyl ether and toxic, corrosive boron trifluoride hydrates. Immediately hydrolyzed by moisture in air to form hydrogen fluoride, BF₃OEt₂ is a fuming liquid with a sharp pungent odour. Upon exposure to water BF₃OEt₂ may emit flammable and corrosive vapors. BF₃OEt₂ may be corrosive to skin, eyes and mucous membranes. Short exposure could cause serious temporary or moderate residual injury. BF₃OEt₂ may be toxic by inhalation. The chemical is incompatible with bases, amines, and alkali metals.

The compound is used as a source of boron trifluoride in many chemical reactions that require a Lewis acid. The compound features tetrahedral boron coordinated to a diethylether ligand. Many analogues are known, including the methanol complex. Traditionally used as a catalyst of organic synthesis, BF₃OEt₂ was frequently-used in acetylation, alkylation, polymerization, dehydration and condensation reaction. BF₃OEt₂ may also be a basic ingredient of producing high quality borohydride fuel and extracting the isotope of boron. Not only used as a catalyst for polybutadiene, polyoxymethylene, coumarone, paints, etc., BF₃OEt₂ was also a curing agent for epoxy resin. Besides, BF₃OEt₂ was a common analytical reagent.

Produced by heating the sulfuric acid, calcium fluoride (fluorite) and boric acid together, boron trifluoride gas reacts with ether boron to produce the trifluoride etherate crude product. The crude complex compound could be obtained by vacuum distillation. The consumption of raw material is as followed: boric acid (≥98%), 560kg/t; calcium fluoride(≥90%) 1150kg/t; fuming sulfuric acid (104.5%), 4100kg/t; ether(≥99%) 725kg/t. Refining would be conducted to get the finished product.