The sunspot, which is 129,000 kilometres across, is known as AR 12192. It rotated into view on Oct. 18, NASA says.
It was clearly visible in many photographs of the Oct. 23 partial solar eclipse.
Sunspots are active regions of the sun where solar explosions called solar flares and coronal mass ejections often erupt. The spots appear dark because they are cooler than other parts of the sun's surface.
Not only is the new giant sunspot is the largest of the current solar cycle that began in 2008, but according to NOAA's Space Weather Prediction Center in Boulder, Colo., it is also the largest active region observed on the sun since November 1990.
Since the beginning of the week, the giant sunspot has produced 27 C-class solar flares, nine M-class solar flares and two X-class flares — the most powerful kind, according to spaceweather.com, a website run by NASA's Tony Philips.
However none of the solar flares, which are local eruptions, have been accompanied by a coronal mass ejection – a blast of charged particles that is flung into space and can hit the Earth if pointed in our direction. Those can interact with our atmosphere to trigger geomagnetic storms that disrupt power grids and satellite communications, as well as generate beautiful auroras.
"It remains a potent force in both area and complexity and is favourably positioned for generating both geomagnetic and solar radiation storms," said the Space Weather Prediction Center in an update posted online Friday. "Is this a sleeping giant or a region lying in wait to hit us with more fury?"
The centre says its forecasters remain "vigilant" and will be posting regular updates.
While this sunspot is the biggest in a long time, it's no record-breaker — it is just a third of the size of the biggest sunspot ever recorded, which appeared in 1947, NASA says.