The documents undermine critics' claims that Pius - condemned by critics as "Hitler's Pope" - put the interests of Rome first and did not protest about the fate of Jews during the Holocaust.
A letter, signed by the Pope in October 1940 and sent to Giuseppe Palatucci, Bishop of Campagna in southern Italy, instructed him to give money "in aid to interned Jews", to whom Pius also referred in an earlier letter as "suffering for reasons of race".
The bishop was already involved in assisting Jews through his nephew, Giovanni Palatucci, the police chief in Fiume, in north-eastern Italy. Palatucci had distributed false identity papers to 5,000 Croatian Jews, enabling them to leave local internment camps for relative safety in his uncle's southern Italian diocese, an operation that would later lead to the police chief's death in Dachau.
A second letter to Bishop Palatucci in November 1940 contained a cheque for 10,000 lira that was to be used for the "support of Jews interned in your diocese".
Supporters believe that the letters will help to repair the reputation of a man whom the present Pope, John Paul II, is seeking to make a saint but who has been accused of being anti-semitic, culturally Germanophile, rabidly anti-communist and conspicuously silent about the fate of Europe's Jews.
"They appear to give compelling proof that will testify to Pius's attitude towards the Jews," said William Doino, an authority on Pius XII.
"Given the dangers then existing and the reluctance of the Church to put such matters in writing, these letters are remarkable. They establish beyond question that Pius XII took a direct, personal interest in helping Jews [and] did so very early on in the war.
"Numerous authors have maintained that there is no credible written evidence that Pius XII himself ever gave direct orders to assist persecuted Jews. Now, we have that evidence."
Mr Doino believes that other new documents to be released by the Vatican to scholars this weekend will shed light on one of the most controversial figures in the Catholic Church's history.
They cover between 1922 and 1939, when the then Eugenio Pacelli was nuncio to Weimar Germany and later papal secretary of state when Hitler came to power.
Controversy has long dogged Pacelli, the principal architect of the 1933 concordat between Germany and the Vatican which ring-fenced Catholic schooling and public worship in a climate that was hostile to "political Catholicism".
While Britain, France and Italy had already established relations with Germany, the concordat is widely viewed as having conferred "respectability" on the Nazis.
The alleged silence of Pius, who became Pope a few months before the outbreak of hostilities in 1939, would later become a stick with which to beat the Catholic Church over its wartime record.
While more than 80 per cent of Italy's Jewish population was rescued, critics of the Church have claimed that individual Italian Catholics acted spontaneously, without aid from Pius XII.
Defenders of Pius XII say he detested the Nazis, signed the concordat to protect German Catholics and put German conservatives who were plotting to kill Hitler in touch with the British, who failed to take much interest in them.
17 September 1999: Wartime Pope 'was pawn of Hitler'
27 February 1999: Pope Pius XII 'wrote attack on Holocaust'
Exposing the myth of Pius XII's 'silence' [7 Sep '02] - PetersNet
Pius XII [Pope biographies] - Encylopedia.com
Pius XI [Pope biographies] - Encyclopedia.com
Concordat between the Holy See and the German Reich - New Advent
Holy See [The official Vatican web-site]
Simon Wiesenthal Centre