Ossoliński National Institute (Polish: Zakład Narodowy im. Ossolińskich, ZNiO), or the Ossolineum is a Polish cultural foundation, publishing house, archival institute and a research centre of national significance founded in 1817 in Lwów (now Lviv). Located in the city of Wrocław since 1947, it is the second largest institution of its kind in Poland after the ancient Jagiellonian Library in Kraków. Its publishing arm is the oldest continuous imprint in Polish since the early 19th century. It bears the name of its founder, Polish nobleman, Count Józef Maksymilian Ossoliński (1748-1826).

Although its origin may be traced to the foreign imposed partitions of the Polish–Lithuanian Commonwealth in the 18th century, the institute's actual history dates from 1817 in the former Polish city of Lwów, then known as Lemberg, capital of Galicia, a province of Austria-Hungary (now Lviv in western Ukraine). The institute first opened its doors to the public in 1817. Ossoliński's purpose was not only to establish a library with archival and other historical materials and a printing works as a basis for the most important Polish national cultural institution at a time when sovereign Poland did not exist, but also to disseminate information to the Polish population through publishing as a means of maintaining cultural ties.

Due to continued existential ordeals provoked by two world wars and other military and political conflicts, such as the ethnic cleansing of the Polish population of the Eastern Borderlands (Kresy) after the annexation of one-third of Poland's landmass in 1939, much of the library and other collections were plundered, scattered or deliberately destroyed. Barely one-third of the artefacts and printed items from the Ossolineum made it to Wrocław after World War II. However those items which survived, did so thanks to the heroic dedication of staff, not least, Mieczysław Gębarowicz.

Since its westerly relocation in 1947 the Ossolineum's Polish department has become the most extensive in the country as it strives to complete a record of the whole Polish scientific and literary oeuvre. It is the repository of manuscripts of some of the foremost Polish scientists, writers and poets, including: Nicolaus Copernicus' De revolutionibus orbium coelestium, Jan Kochanowski, Adam Mickiewicz, Adam Asnyk, Jan Kasprowicz, Władysław Reymont, Stefan Żeromski, Juliusz Słowacki, and in particular Henryk Sienkiewicz, whose chief publisher it has been.

The ZNiO departments are: the Ossolineum Library (1816), the Lubomirski Museum (1823), Ossolineum Publishers (1827), the Pan Tadeusz Museum.

Ossoliński who was a politician, writer and researcher had already devoted his long life to building and cataloguing an extremely rich library collection, consisting of books, manuscripts, prints and coins. It was only upon mature consideration and after observing developments since the Congress of Vienna that he opted for Lwów, as the most suitable place in which to house his 52 crates of materials having obtained prior approval of the Austrian emperor, Francis I. Thus began his Institute in Lwów (Institut in Lemberg in German). The income from Ossoliński's landed properties served for over three decades to finance his acquisitions and collection. The Ossolineum quickly became a celebrated centre for Polish science and culture which not only survived under foreign rule, but throughout the Second Polish Republic in the Interwar period.

Until the 1939 invasion of Poland it had combined a library, publishing house, and the Lubomirski Museum. The National Ossoliński Institute had been located from its foundation until 1945 in the former convent and church buildings of the Carmelite Order of nuns in Lwów at 2, Ossolińscy street, (since renamed vulytsia Stefanyka). After the first partition of Poland and the dissolution of many convents by the Austrian emperor Joseph II, the convent buildings had become a ruin. Their restoration became a project for General Józef Bem who in 1823 merged the Lubomirski Museum (originally founded by Prince Henryk Lubomirski) with the Ossoliński Institute.

Under Austrian rule, the Ossolineum became a beacon for the Polish independence movement and was one of the most important centres of Polish culture despite foreign rule and the Germanization of its structures. During that time there were many persecutions such as police searches and arrests of employees of the institute. It housed a clandestine Polish printing works in the early 1840s, and had exclusive rights for publishing textbooks under the relative Galician autonomy. During the revolutionary upheaval in 1848, the Ossolineum became a Polish landmark in an otherwise highly ethnically diverse city.