Andrzej Duda took part in a ceremony at a Polish military cemetery at Bykivnya in the Ukrainian capital, Kiev, to mark the 1940 mass murders.
He said that "the truth about the Katyn massacre, about the bestial genocidal murders that the Soviets carried out on Polish officers in Katyn, in Kharkiv and here in Bykivnya, is today one of the strongest foundations of a free, truly sovereign, truly independent Poland."
Around 22,000 Polish prisoners of war were killed in the spring of 1940 on orders from top Soviet authorities.
Following the Soviet invasion of Poland in September 1939, thousands of Polish officers were deported to camps in the Soviet Union.
POWs from camps in Kozelsk, Starobelsk and Ostashkov as well as Poles held in prisons run by the Soviet Union's NKVD secret police were among those murdered in April 1940.
The Soviets for decades denied they had carried out the massacres, while the topic was taboo in Poland while the country was ruled by a Moscow-backed communist regime.
(pk)