Jews have been seeking the move for decades.
Some said the pope, who served from 1939 to 1958, turned a blind eye to the Holocaust by not speaking out forcefully.
The Vatican has claimed he worked quietly behind the scenes to save Jews and not to worsen the situation for many, including for Catholics in parts of Nazi-occupied Europe.
Pope Francis declared that “the church isn’t afraid of history” and said Pius XII’s legacy had been treated with “some prejudice and exaggeration”.
He told employees of the Vatican Secret Archives that documents spanning 1939 to 1958 would be open to researchers on 2 March 2020.
The Vatican typically waits 70 years after the end of a pontificate to open archives.
It has been under pressure to make the Pius XII documents available sooner, while Holocaust survivors are still alive.
Pius XII’s actions will be scrutinised as part of efforts to decide if he should be made a saint.
One of the world’s leading Jewish groups, the American Jewish Committee (AJC), welcomed the move.
“For more than 30 years, the AJC has called for the full opening of the Holy See’s Secret Archives from the period of World War Two,” said Rabbi David Rosen, the AJC’s international director of interreligious affairs.
“It is particularly important that experts from the leading Holocaust memorial institutes in Israel and the US objectively evaluate as best as possible the historical record of that most terrible of times, to acknowledge both the failures as well as the valiant efforts made during the period of the Shoah,” he said, using the Hebrew word for the Holocaust.
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