Andrzej Duda signed the proposal into effect after it passed both houses of parliament, state news agency PAP reported.
Deputy Prime Minister Władysław Kosiniak-Kamysz on Thursday welcomed the president's signature, calling it another step in honouring the memory of those murdered.
He said they "died because they were Poles."
Kosiniak-Kamysz, whose rural-based Polish People's Party (PSL) drafted the bill, said last month that Polish-Ukrainian relations can only be built on historical truth.
"July 11 will be the Day of Remembrance for Poles — victims of the genocide committed by OUN-UPA on the Eastern Borderlands of the Second Polish Republic," Kosiniak-Kamysz wrote on X at the time.
“We honour memory and strengthen identity because only on historical truth can we build lasting mutual relations," he added.
The law signed by the president designates July 11 as a national holiday commemorating the mass killings of ethnic Poles by the Organisation of Ukrainian Nationalists (OUN), the Ukrainian Insurgent Army (UPA) and other Ukrainian nationalist groups.
The groups were active between 1939 and 1946 in what was then eastern Poland and is now western Ukraine.
More than 100,000 ethnic Poles, primarily rural residents, were murdered in the massacres, which reached their peak in July 1943.
July 11, 1943, known as "Bloody Sunday," marked the worst day of the atrocities when coordinated attacks were carried out in nearly 100 towns and villages.
Polish civilian victims of a World War II massacre committed by the Ukrainian Insurgent Army (UPA). Image: Władysława Siemaszków, Ludobójstwo, page 1294, from Henryk Słowiński collection, Public domain, via Wikimedia Commons
The law says the victims "suffered martyrdom solely because of their Polish identity," and affirms that they deserve national remembrance through an annually observed national holiday.
The violence took place in the Volhynia region and the neighbouring provinces of Lwów, Tarnopol and Stanisławów—areas that were once part of Poland’s "Eastern Borderlands" before borders shifted after WWII.
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Source: IAR, PAP