涡轮扇发动机（亦称涡扇发动机或者内外函喷气发动机；英文：Turbofan Engine）是一种燃气涡轮式航空发动机，主要特点是其首级扇叶的面积大过涡轮喷射发动机的首级扇叶很多，部分空气经过的涡轮喷气发动机称为内涵道，经由内涵的涡轮驱动首级增压扇叶推动空气经过的外侧部分称为外涵道，增压扇叶同时具有螺旋桨和压缩空气的用途的作用，能将部分吸入的空气通过喷射发动机的外围提供直接推力，内外涵共同产生推力。可同时具有涡轮螺旋桨与涡轮喷射推进发动机的双重推力供给。

涡轮扇发动机并非只依靠涡轮喷射直接燃油燃烧出高压空气提供推力，所以单位推力小时耗油率比纯涡轮喷射佳，但螺旋扇叶的推力重量比耗损功率较大，比较纯涡轮喷射，超音速飞行时其受到边界层影响使其低气压高空与超音速下推力重量不佳。涡扇发动机最适合飞行速度为每小时400至2,000公里时使用，故此现在多数的喷射机发动机都是采用涡扇发动机作为动力来源。

The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which takes mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to accelerate air rearwards. Thus, whereas all the air taken in by a turbojet passes through the turbine (through the combustion chamber), in a turbofan some of that air bypasses the turbine. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of those contributing to the thrust. The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working in concert; engines that use more jet thrust relative to fan thrust are known as low bypass turbofans, conversely those that have considerably more fan thrust than jet thrust are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass. Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.