The article is devoted to the person of Achilles de Grassis, from Bologna, Cardinal Protector of Poland at the Holy See. After completing his law studies, this official of the Roman Curia became bishop of Cittä di Castello in 1506. On behalf of Pope Julius II, he performed several important diplomatic missions, including: to the French king Louis XII. and Emperor Maximilian I. In the autumn of 1510, he stayed in Poland as legate de latere, where he was recruited by King Sigismund I to promote Polish affairs in the Curia. In March 1511 Julius II. appointed him cardinal, and a few months later bishop of Bologna. In the autumn of the same year, he succeeded the deceased Cardinal Isvaljez as protector of Poland, a position he held until his death. At that time, he was involved in many political and charitable matters, the most important of which were problems related to the organization of the anti-Turkish crusade and the appointment of episcopal sees. His most active activity took place in the years 1512-1517, where his main task was to support the Polish side before the forum of the Fifth Lateran Council in its dispute with the Teutonic Knights. He also informed the Polish king about all important events at the papal court and took care of the Polish envoys during their stay in Rome. With the death of Achilles de Grassis in 1523, Sigismund I. lost a trusted advocate of Polish interests in the Roman Curia.