It was Poltoranin's second cross-country skiing World Cup win of his career.
The 25-year-old claimed his one and only win in a 15km classic in Davos in December of 2010, while he was second in this season's opener in Gaellivare.
Poltoranin was also third overall in the Ruka Triple and those good performances came to fruition in La Clusaz as the Kazakhstan skier won a tight 15km classic mass start.
However it was a triumph that should have never been. Russia's Bessmertnykh, leading on the final sprint, held his arms aloft in celebration before realising how close Poltoranin was and lunging for the line.
He was a fraction too late, losing out by just 0.01 seconds as Poltoranin clocked a time of 37 minutes 11.6 seconds.
Just 1.8s separated the top six with defending World Cup champion Dario Cologna crossing the finishing line just 0.4s shy of the top two for third while Didrik Toenseth was fourth, a further 0.6s back.
Alexander Legkov, who won this season's Tour de Ski, came home 0.4s behind for fifth with German Hannes Dotzler sixth by the same margin from the Russian.
Cologna narrowed the lead on Legkov at the top of the overall World Cup standings to 41 points with third with Petter Northug still third despite choosing to skip the race.