Scientists working with NASA's Solar Dynamics Observatory, a space telescope that constantly scans the sun, said the erupting flare reached peak brightness at 11:22 p.m. EDT Monday.
Three strong flares had already erupted from the same sunspot before the rotation of the sun brought it into view from the Earth, they said.
"This means more flares are probably in the offing, and they will become increasingly Earth-directed as the sunspot turns toward our planet in the days ahead," astronomer Tony Phillips wrote on Spaceweather.com.
The Solar Dynamics Observatory captured the erupting flare in still images and video, SPACE.com reported.
Astronomers classify the strength of solar flares in terms of energy release, with Monday's flare ranked as an X-class event, the most powerful level.
Solar activity increased and decreases on an 11-year cycle, with a peak of magnetic activity expected in 2013.