The probed lifted off atop a Delta IV Heavy rocket at 3:31 am (0731 GMT). A last-minute technical glitch on Saturday had postponed the launch by 24 hours.
The spacecraft, about the size of a small car, will travel directly into the sun's atmosphere, where the temperature is about 1,300 degrees Celsius, to become the first probe to reach the sun's corona.
Scientists hope it will unlock mysteries of the sun, such as why the corona is hotter than the surface of the sun itself. They also hope to learn more about the sun's magnetic field and solar wind.