Russian involvement in the Pacific consists of two relatively brief episodes that are essentially book ends to the war because of the Soviet-Japanese Neutrality Pact of April 1941 which was eventually scrapped by the Soviet Union in August of 1945.

In 1939 the Soviets and Japanese fought a series of brief but costly battles over the border between the Japanese client state of Manchuko and Soviet Mongolia. The Soviets won but neither power was particularly enamored with the idea of another conflict; Russia was concerned with the growing threat of a resurgent Germany and Japan was seeing a rapid breakdown in its relationship with the United States that was seen as headed to open war. The pact was signed in April and Germany launched Operation Barbarossa in June. The pact held throughout the German invasion, freeing up critical troops to halt the German advance. There's speculation that Japan would have invaded Siberia if the Germans had been more successful but that's mostly speculative based on circumstantial evidence (a very talented Japanese field commander was reassigned to Siberia, where he'd be most useful in the event of an invasion but not very useful as the commander of a sleepy border army).

Edit: Less circumstantial than I recalled. Basically with the outbreak of war in Europe, Japan had to choose between going north against the Russians, which the Germans preferred for obvious reasons, and going south to secure the oil in the Dutch East Indies while simultaneously snapping up the colonial possessions of the European empires that Hitler was defeating. Japan imported something like 80% of their oil from the U.S. and had converted their fleet from coal to oil so they'd run out of literal fuel for their war machine in a year or so if the U.S. cut supplies, which the Japanese were afraid they'd do (and the Americans had internally discussed to force Japan to withdraw from China). The U.S. oil embargo in the wake of Japan's seizure of French Indochina made war with the Soviets impossible and seizure of the Dutch East Indies imperative. Ironically, the Americans were aware of this and had explicitly avoided cutting the oil supply while simultaneously working hard to convince the Japanese that any aggression against Dutch or British enclaves would be met with force (including moving their Pacific fleet from their home bases on the U.S. West Coast to their forward operating bases at Pearl Harbor). The Americans had no intention of entering the war if only Dutch or British installations were attacked because the American people wouldn't support it. The Japanese were successfully fooled though, assumed war work the U.S. was inevitable, concluded they were unlikely to win under the best of circumstances but that the odds got worse every day (because they had less fuel and European/American defenses were being strengthened daily at that point), and the rest is history.

The pact held until August 1945, when the Soviets notified Japan that a state of war would exist between their countries on August 9. The Soviets had agreed at Yalta to enter the war against Japan within three months of the German surrender in exchange for territorial concessions. The Japanese had been hoping that the Soviets would mediate between the Allies and Japan to reach surrender terms that allowed Japan to retain some of their territorial gains or at least be more favorable than the unconditional surrender demand by the Allies. Japanese defense were focused on stalling an American invasion from the south so when the Soviets crushed the Japanese armies holding the mainland enclaves Japanese strategic thinking had to be rapidly adjusted. The Japanese surrender followed shortly, driven in part by the closing off of the Soviet avenue in conjunction with the American atomic bombings.

Edit: JSTOR is free right now so you can read a lot of articles about the buildup to war in Japan. If you really have a lot of free time these days "Japan Prepares for Total War" by Michael Barnhart is pretty detailed.