The state of war between Japan and Allied Powers was terminated at the signing of San Francisco Treaty of 1951.
USSR refused to sign the treaty, due to dispute over Kuril Islands (which were promised to USSR during Yalta Conference, but not Potsdam, and the treaty was based on Potsdam for reasons discussed on Wikipedia)
The U.S. maintains that until a peace treaty between Japan and Russia is concluded, the disputed Northern Territories remain Japanese territory under Russian control via General Order No. 1. (reference: Bruce A. Elleman, Michael R. Nichols and Matthew J. Ouimet, A Historical Reevaluation of America's Role in the Kuril Islands Dispute, Pacific Affairs, Vol. 71, No. 4 (Winter, 1998-1999), pp. 489-504)
This Asia Times article has a good look at the issue of Kurils.
As of 2015, the issue is still unsettled and therefore Russia and Japan are not in a state of peace yet.