During his life, Wojciech Kętrzyński was a renowned and valued historian, librarian and publicist. Many obituaries and commemorations appeared in magazines and academic journals after his death. The employees of the Lviv Ossolineum made sure to preserve the memory of their director also outside the town. In Lviv, Kętrzyński has had a street and one of the reading halls in Ossolineum named after him. His poems and memoir were pub-lished, along with some commemorations dedicated to his achievements. The memory of Kętrzyński has also lasted in southern parts of Eastern Prussia. Michał Kajka translated his poems into Polish, while Emilia Sukerto-wa-Biedrawina in Działdowo published articles about Kętrzyński in her calendars. After 1945, as Polish borders encompassed those parts of Eastern Prussia, Kętrzyński became a reclaimant, even a warrior of Polishness. In his honour, the town of Rastenburg was renamed to Kętrzyn. He has had streets and schools named after him. The research on Kętrzyński’s activities gained momentum with the establishment of the Wojciech Kętrzyński Centre for Scientific Research (Pol. Ośrodek Badań Naukowych, OBN) in Olsztyn. Thanks to the efforts of OBN, a Polish plaque appeared at Kętrzyński’s grave, which was found by Leonard Turkowski in 1969 at the Lychakiv Cemetery in Lviv. In 2008, the tombstone was renovated as a result of the activity of Kętrzyn authorities. When the old pre-War fragment of the tombstone was found in 2016, it was returned to its proper place, while the medallion with Kętrzyński’s image was gifted by Kętrzyn authorities to the Wrocław Ossolineum. The memory of Kętrzyński in Warmia and Masuria was preserved by publishing his poems and research articles O ludności polskiej w Prusiech niegdyś krzyżackich [Eng. On Polish people in the previously Teutonic Prussia]. Numerous academic conferences confirmed the current nature of Kętrzyński’s conclusions. The Marshall of the Warmińsko-Mazurskie voivode-ship established an all-Poland award in humanities named after Kętrzyński, contributing to the movement of commemorating the researcher in the region and in Poland. This paper summarises all such activities during the 100-years period since Wojciech Kętrzyński’s death.