The resolution - backed by 432 deputies, with no votes against and 10 deputies abstaining - refers to the Volhynia Massacre, a black page in Polish-Ukrainian relations.
Ukrainian President Petro Poroshenko voiced regret at the decision by Polish MPs.
On 11 July 1943, the Ukrainian Insurgent Army (UPA) carried out a coordinated attack on some 100 villages largely inhabited by Poles in Eastern Galicia and in Volhynia, which is currently in western Ukraine.
The Volhynia region, which was within Polish borders prior to World War II, was first occupied by the Soviets in 1939, and then by the Nazi Germans in 1941.
Some 100,000 ethnic Poles in total were slaughtered in the 1940s by Ukrainian forces, according to some estimates.
"The victims of crimes committed in the 40s by Ukrainian nationalists have so far not been commemorated in an appropriate manner and the mass murders have not been named - in keeping with historical truth - as genocide," reads the resolution adopted by Polish MPs.
The resolution added: "As a result of genocide perpetrated 1943-1945, over 100,000 citizens of the [Polish] Second Republic were murdered, mainly peasants. Their exact number is still not known, and many of them have not yet had a dignified burial and commemoration."
In the resolution, which also paid tribute to those Ukrainians who rescued Poles, Polish MPs also expressed "solidarity with Ukraine as it struggles with external aggression to preserve its territorial integrity."
Ukrainian President Poroshenko voiced concern that the Polish MPs' resolution referring to genocide could be used for "political speculation."
"Only by taking joint steps, can we come to Christian agreement and reconciliation. Only together can we shed light on all the facts of the tragic pages of our common history," he said.
Earlier this month, Poroshenko paid tribute to the victims of the massacres at a commemorative monument in Warsaw.