They'd had some border skirmishes before WWII even started, with tens of thousands of casualties on each side. The 1941 neutrality pact was a result of a combined Soviet/Mongolian force defeating and removing the Japanese from Mongolia.
I'm still dubious. Adding a new theater to the war when your other theaters are already stalling without a decisive victory is generally not a winning measure. The naval victory is only at best a temporary reprieve: the US is going to replace all its lost carriers within a year, and is similarly going to replace all the capital units [1] it lost at Pearl Harbor by that time. The Japanese didn't think themselves capable of mounting an invasion of the USSR before about mid-1943, by which point it was beginning to feel pressure on other fronts, and the IJA would start raiding the Manchurian army groups for manpower to keep from losing on the fronts they were already engaged in.
[1] Again, recall that to Japanese strategic thinking at this time, it's the battleship strength that matters. Carriers are just a sideshow.
Japan and the Soviet Union did not go to war with each other until August 1945.