Ottoman Turkish (لسان عثمانی, Lisân-ı Osmânî, Turkish pronunciation: [liˈsaːnɯ osˈmaːniː]; Turkish: Osmanlıca) was the standardized register of the Turkish language in the Ottoman Empire (14th to 20th centuries CE). It borrowed extensively, in all aspects, from Arabic and Persian. It was written in the Ottoman Turkish alphabet. Ottoman Turkish was largely unintelligible to rural Turks, who continued to use kaba Türkçe ("raw/vulgar Turkish"; compare Vulgar Latin and Demotic Greek), which used far fewer foreign loanwords and is the basis of the modern standard. The Tanzimât era (1839–1876) saw the application of the term "Ottoman" when referring to the language (لسان عثمانی lisân-ı Osmânî or عثمانلیجه Osmanlıca); Modern Turkish uses the same terms when referring to the language of that era (Osmanlıca and Osmanlı Türkçesi). More generically, the Turkish language was called تركچه Türkçe or تركی Türkî "Turkish".

Historically, Ottoman Turkish was transformed in three eras:

In 1928, following the fall of the Ottoman Empire after World War I and the establishment of the Republic of Turkey, widespread language reforms (a part in the greater framework of Atatürk's reforms) instituted by Mustafa Kemal Atatürk saw the replacement of many Persian and Arabic origin loanwords in the language with their Turkish equivalents. One of the main supporters of the reform was the Turkish nationalist Ziya Gökalp. It also saw the replacement of the Perso-Arabic script with the extended Latin alphabet. The changes were meant to encourage the growth of a new variety of written Turkish that more closely reflected the spoken vernacular and to foster a new variety of spoken Turkish that reinforced Turkey's new national identity as being a post-Ottoman state.^{[citation needed]}

See the list of replaced loanwords in Turkish for more examples of Ottoman Turkish words and their modern Turkish counterparts. Two examples of Arabic and two of Persian loanwords are found below.

Historically speaking, Ottoman Turkish is the predecessor of modern Turkish. However, the standard Turkish of today is essentially Türkiye Türkçesi (Turkish of Turkey) as written in the Latin alphabet and with an abundance of neologisms added, which means there are now far fewer loan words from other languages, and Ottoman Turkish was not instantly transformed into the Turkish of today. At first, it was only the script that was changed, and while some households continued to use the Arabic system in private, most of the Turkish population was illiterate at the time, making the switch to the Latin alphabet much easier. Then, loan words were taken out, and new words fitting the growing amount of technology were introduced. Until the 1960s, Ottoman Turkish was at least partially intelligible with the Turkish of that day. One major difference between Ottoman Turkish and modern Turkish is the latter's abandonment of compound word formation according to Arabic and Persian grammar rules. The usage of such phrases still exists in modern Turkish but only to a very limited extent and usually in specialist contexts; for example, the Persian-derived genitive construction تقدیر الهی (takdîr-i ilâhî) (which reads literally as "the preordaining of the divine" and translates as "divine dispensation" or "destiny") is used, as opposed to the normative modern Turkish construction, ilâhî takdîr (literally, "divine preordaining").

In 2014, Turkey's Education Council decided that Ottoman Turkish should be taught in Islamic high schools and as an elective in other schools, a decision backed by President Recep Tayyip Erdoğan, who said the language should be taught in schools so younger generations do not lose touch with their cultural heritage.

Most Ottoman Turkish was written in the Ottoman Turkish alphabet (Ottoman Turkish: الفبا, romanized: elifbâ), a variant of the Perso-Arabic script. The Armenian, Greek and Rashi script of Hebrew were sometimes used by Armenians, Greeks and Jews. (See Karamanli Turkish, a dialect of Ottoman written in the Greek script; Armeno-Turkish alphabet)

The actual grammar of Ottoman Turkish is not different from the grammar of modern Turkish. The focus of this section is on the Ottoman orthography; the conventions surrounding how the orthography interacted and dealt with grammatical morphemes related to conjugations, cases, pronouns, etc.