The rider, who will join Saur-Sojasun for 2013, won the 202.9km race which is run between Ham and Albert.
The 23-year-old Lithuanian escaped from a leading group of 13 riders and stayed out to the end in the Somme.
It is Siskevicius's third win of the year, having taken stages in the Tour de Bretagne and Tour du Lumousin.
1. Evaldas Siskevicius (Lithuania / La Pomme Marseille)
2. Clement Koretzky (France / La Pomme Marseille)
3. Mickael Delage (France / Française des Jeux)