Pranciškus Petras Būčys (, 20 August 1872 – 25 October 1951) was a Lithuanian Roman Catholic priest, university professor, titular bishop of the Eastern Catholic Church (consecrated in 1930), and Superior-General of the Congregation of Marian Fathers of the Immaculate Conception (1927–1933, 1939–1951). Born to a Lithuanian peasant family active in book smuggling, he studied at the Marijampolė Gymnasium and Sejny Priest Seminary and was active in Lithuanian cultural life. He started contributing articles to Lithuanian press, including Vienybė lietuvninkų and Varpas, in 1891. He continued his studies for a master's degree at the Saint Petersburg Roman Catholic Theological Academy where he formed a life-long friendship with fellow cleric Jurgis Matulaitis-Matulevičius. He earned his doctorate in theology at the University of Fribourg in Switzerland in 1901. In 1902, he became professor of apologetics and fundamental theology at the Saint Petersburg Roman Catholic Theological Academy. During the Russian Revolution of 1905, he participated in the Great Seimas of Vilnius and helped draft the program of the Lithuanian Christian Democratic Party. Bučys was prorector and acting rector of the academy in 1912–1915, but resigned upon learning that he would not be promoted to rectors because he was not Polish. Together with Matulaitis-Matulevičius, Bučys joined the Congregation of Marian Fathers of the Immaculate Conception in 1909. At the time, the congregation was reduced to a single elderly member. Matulaitis-Matulevičius and Bučys spent considerable time and effort working on reviving and expanding the congregation. In 1916, Bučys traveled to United States to work with the newly established Marian Fathers in Chicago. He served as pastor to Lithuanian parishes in Sheboygan, Wisconsin, and Waukegan, Illinois, and edited the struggling Lithuanian daily Draugas. In 1921, he returned to Lithuania and worked on organizing the Faculty of Theology at the new University of Lithuania. He was dean of the faculty in 1922–1923 and 1925–1926, as well as university prorector in 1923–1924 and rector in 1924–1925. After the death of Matulaitis-Matulevičius in 1927, Bučys was elected as the Superior-General of the Marian Fathers and moved to Rome. There he became an advisory member of the pontifical commission on Russia and was consecrated as titular bishop of Olympos on 6 July 1930. He was tasked with a Catholic mission to convert Eastern Orthodoxs and Old Believers to Eastern Catholicism. He visited Russian diaspora in Europe and United States working to organize parishes. In 1934, he was ordered to work