The second dictionary in which Turkish words were made headwords in the Ottoman period is an undiscovered dictionary titled as Mir‟atü‟l-Müşkil. In this sense, the dictionary predates the work of Mehmed Esad Efendi titled Lehcetü‟l-Lügat which is known as the first active dictionary of the Ottoman period. The work which was organized trilingual as Turkish-Arabic-Persian was written by the famous poet of seventeen century, Osmanzâde Ahmed Tâib. Among the written works of Osmanzâde Ahmed Tâib who is known as “reîs-i şâirân” in his period, there weren‟t any dictionaries with this name in the sources. Mir‟atü‟l-Müşkil is important since it is the second active dictionary of Turkish lexicography in the Ottoman period as well as being an unknown work of Osmanzâde Ahmed Tâib. Mir‟atü‟l-Müşkil which is quite valuable for Turkish language and Turkish literature history was introduced the first time with this research. Information on the name of the work, its writer, copyright date, copy, editing and Turkish vocabulary were included in the article and the transcription of the work‟s introduction was presented.