Japan nearly went to war with the USSR in 1941 when Germany invaded and it looked as if the USSR was near collapse. The Japanese general staff had begun to move troops into Manchuria and they began to formulate a plan of attack. What stopped them was the fact that the Soviets were putting up fierce resistance and the collapse of the USSR, that Germany had assured Japan was about to happen any second, was not materializing. Ultimately, the plan to attack the USSR was shelved in favour of an attack against the USA, and the other Western nations in East Asia.

Now the Soviet Union didn't declare war on the Japanese because they were occupied with the German invasion. The Soviets kept a sizable garrison on the Japanese border, but once they were sure that the Japanese were not going to attack they transferred many of these units to the German front (I should point out that this point is debated among historians). The Soviets didn't need another enemy when they already had a huge one in Germany.

Now, the Soviet Union did end up declaring war against the Japanese in August. There were three reasons behind this. One, with the German surrender the Soviets had no other enemies and thus could afford to fight Japan completely, two, the western allies had asked for the Soviets to invade Japan at the Yalta conference and Stalin agreed to do so no later than 90 days after the surrender of Germany, finally, Stalin wanted a say in any peace deal in the Far East and the best way to achieve that was through having a military presence. The invasion allowed the Soviets to set up a communist state in North Korea, give Manchuria to the communist Chinese (after they stripped it of important materials) and allowed them to annex the Sakhalin and Kukril isles.

Source:

Third Reich at War by Richard Evans

Japan's Imperial Army by Edward Drea

The Second World War by John Keegan