Effendi or effendy (Turkish: efendi [eˈfændi]; Ottoman Turkish: افندی, romanized: afandi; originally from Medieval Greek: αφέντης [aˈfendis]) is a title of nobility meaning sir, lord or master, especially in the Ottoman Empire and the Caucasus. The title itself and its other forms are originally derived from Medieval Greek aphentēs which is derived from Ancient Greek authentēs meaning lord.

It is a title of respect or courtesy, equivalent to the English Sir. It was used in the Ottoman Empire and Byzantine Empire. It follows the personal name, when it is used, and is generally given to members of the learned professions and to government officials who have high ranks, such as bey or pasha. It may also indicate a definite office, as hekim efendi, chief physician to the sultan. The possessive form efendim (my master) was formerly used by slaves, and is commonplace in formal discourse, when answering the telephone, and can substitute for "excuse me" in some situations (e.g. asking someone to repeat something).

In the Ottoman era, the most common title affixed to a personal name after that of agha was efendi. Such a title would have indicated an "educated gentleman", hence by implication a graduate of a secular state school (rüşdiye), even though at least some if not most of these efendis had once been religious students, or even religious teachers.

Lucy Mary Jane Garnett wrote in the 1904 work Turkish Life in Town and Country that Ottoman Christians, women, mullahs, sheiks, and princes of the Ottoman royal family could become effendi, a title carrying "the same significance as the French Monsieur" and which was one of two "merely conventional designations as indefinite as our 'Esquire' has come to be [in the United Kingdom]".

The Republican Turkish authorities abolished the title circa the 1930s.