The ruling, issued by the Constitutional Court in a unanimous decision from its headquarters in St Petersburg, clears the way for a final parliamentary vote to ratify entry into the 155-member global trade rules club.
The vote will take place on Tuesday with a majority of lawmakers expected to rubber-stamp accession. The original deal was clinched last December after 18 years of often-difficult talks.
Russia, whose $1.9 trillion economy is the largest outside the WTO, would become a full member 30 days after ratification.
The court's ruling quashed a case brought by lawmakers from the opposition Communist and Just Russia parties who had unsuccessfully argued that the ratification procedure and parts of the accession deal were unconstitutional.
Recently elected for a third presidential term, President Vladimir Putin had long appeared ambivalent over WTO entry but warmed to the process after Russia's economy was hit hard by the global recession of 2008-2009.
According to a World Bank study, the growth uplift that Russia could expect from joining the WTO could be 3.3% over the medium term and as much as 11% in the long run.
Under the deal, Russia would gradually cut average import tariffs to 7.8% from 10% and open up investment in sectors such as telecommunications, while shielding its banking sector from overall foreign control.
Russia managed to protect hefty subsidies to promote its domestic auto industry and negotiated a long transitional period for reducing state aid to farmers.
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