The adult male, nicknamed Iceberg, appears to be healthy and leading a normal life in its pod, they said.
While various species of whale occasionally will present a white specimen, the only known white orcas have been young animals, with adults all showing the normal black and white coloration.
The sighting of the adult white orca occurred during a research cruise off Kamchatka by Russian scientists and students.
It was co-led by Erich Hoyt, a long-time orca scientist, conservationist and author.
"We've seen another two white orcas in Russia but they've been young, whereas this is the first time we've seen a mature adult," Hoyt told BBC News. "It has the full two-meter-high (6.5 feet) dorsal fin of a mature male, which means it's at least 16 years old -- in fact the fin is somewhat ragged, so it might be a bit older."
Male orcas can live to 50 or 60 years old, though 30 is more usual.
"Iceberg seems to be fully socialized; we know that these fish-eating orcas stay with their mothers for life, and as far as we can see he's right behind his mother with presumably his brothers next to him," Hoyt said.