The aim of this paper is to investigate and follow the development and operation of the Library of the Royal Grammar School (Velika gimnazija) in Osijek from its formal foundation, in the school year 1851/52, to its closure in 1929. For the purposes of this research, the analysis of the contents of the printed annual reports of the Royal Grammar School, which are in the possession of the Library Department of the Museum of Slavonia, was carried out, as well as the analysis of the archival material of the State Archives in Osijek. Given that, at the time, schools were obliged to provide status reports for the libraries, the relevant reports represent a significant contribution to the continuous monitoring of the increase in the Teachers' and Student Library holdings, as well as an insight into the work of the teachers who served as librarians. Finally, in addition to a clearer insight into the educational, social and historical frameworks in which the secondary education in Osijek developed, the analysis of the archival material and the Grammar School annual reports have also shown that, through-out the observed period, the Grammar School Library operated continuously, the books arrived regularly, either by purchase or donation, and that its holdings contained valuable and rare specimens from various fields of science. Given that today the old library holdings of the former Royal Grammar School are located in the Library Department of the Museum of Slavonia, where they were successfully stored in 1943 during World War II, this certainly gives it the legitimacy of a significant cultural and historical source for further research and interpretation of the cultural and educational history of the city of Osijek. U radu se istražuje i prati razvoj i djelovanje knjižnice Velike gimnazije u Osijeku u razdoblju od njezina formalnoga osnutka, školske godine 1851./52., pa sve do ukidanja Gimnazije 1929. Za potrebe ovoga istraživanja provedena je analiza sadržaja tiskanih godišnjih izvješća Velike gimnazije koji se nalaze u posjedu Odjela knjižnice Muzeja Slavonije te analiza arhivskoga gradiva Državnoga arhiva u Osijeku. S obzirom da su u to vrijeme škole u njima bile dužne navoditi i podatke o stanju samih knjižnica, dotična izvješća predstavljaju značajan doprinos kontinuiranome praćenju rasta učiteljskog i učeničkog fonda, kao i uvid u rad nastavnika koji su bili u službi knjižničara. Napokon, osim jasnijega uvida u obrazovne i društveno-povijesne okvire u kojima se razvijalo osječko srednje školstvo, analiza arhivskoga gradiva i godišnjih izvješća Velike gimnazije pokazala je i da je kroz cijelo to promatrano razdoblje školska knjižnica djelovala u kontinuitetu, da je knjižna građa redovno pristizala kupnjom i darom te da je njen fond ujedno sadržavao i vrlo vrijedne i rijetke primjerke iz različitih područja znanosti. S obzirom da se danas stari fond bivše Velike gimnazije nalazi u Odjelu knjižnice Muzeja Slavonije u koji je uspješno pohranjen tijekom ratne 1943. godine, to mu svakako daje legitimitet ozbiljnoga kulturno-povijesnog izvorišta za daljnja istraživanja i tumačenja kulturne i obrazovne povijesti grada Osijeka.