Amateur Russian skywatchers Artyom Novichonok and Vitali Nevski observed the object dubbed Comet ISON in September, Discovery.com reported.
"The object was slow and had a unique movement," Novichonok wrote on a comet-hunting blog.
"But we could not be certain that it was a comet, because the scale of our images are quite small and the object was very compact," he wrote of the object beyond the orbit of Jupiter.
However, follow-up observations confirmed the discovery.
In late 2013 Comet ISON is expected to pass extremely close to the sun, putting on a light show expected to be visible to the naked eye for skywatchers on Earth from early November through the first few weeks of January 2014.
The comet may be the brightest to appear in the sky since 1965 and may even be visible in daylight, the International Astronomical Union's Minor Planet Center predicted.