The Pratt & Whitney J57 (company designation: JT3C) is an axial-flow turbojet engine developed by Pratt & Whitney in the early 1950s. The J57 (first run January 1950) was the first 10,000 lbf (45 kN) thrust class engine in the United States. It is a two spool engine.

The J57/JT3C was developed into the J52 turbojet, the J75/JT4A turbojet, the JT3D/TF33 turbofan, and the XT57 turboprop (of which only one was built). The J57 and JT3C saw extensive use on fighter jets, jetliners, and bombers for many decades.

The J57 was a development of the Pratt & Whitney XT45 (PT4) turboprop engine that was originally intended for the Boeing XB-52. As the B-52 power requirements grew, the design evolved into a turbojet, the JT3.

Pratt & Whitney designed the J57 to have a relatively high overall pressure ratio to help improve both thrust-specific fuel consumption and specific thrust, but it was known that throttling a single high-pressure-ratio compressor would cause stability problems. As Sir Stanley Hooker explains in his autobiography, the outlet area of a compressor is significantly smaller than that of its inlet, which is fine when operating at the design pressure ratio, but during starting and at low throttle settings the compressor pressure ratio is low so ideally the outlet area should be much larger than its design value. Put crudely the air taken in at the front cannot get out the back, which causes the blades at the front of the compressor to stall and vibrate. The compressor surges, which normally means the airflow reverses direction, causing a sharp drop in thrust.

By the late 1940s, three potential solutions to the stability problem had been identified:

1) bleeding any excess compressed air at part speed overboard through interstage blow-off valves

2) incorporating variable geometry in the first few stages of the compressor

3) splitting the compressor into two units, one of which supercharges the other, with both units being mounted on separate shafts and driven by their own turbine