A gabbai (Hebrew: גבאי), sometimes spelled gabay, also known as shamash (שמש, sometimes transcribed shamas) or warden (UK, similar to churchwarden), is a beadle or sexton, a person who assists in the running of synagogue services in some way. The role may be undertaken on a voluntary or paid basis. A shamash (literally 'servant') or gabbai can also mean an assistant to a rabbi (particularly the secretary or personal assistant to a Hasidic rebbe).

In ma'amad, the Council of Elders (or "the board of directors") of the communities of Sephardi Jews, the position of gabbai was that of the treasurer.

The word gabbai is Hebrew and, in Talmudic times, meant "collector of taxes or charity" or "treasurer".

The term shamash is sometimes used for the gabbai, the caretaker or "man of all work" in a synagogue.

While the specific set of duties vary from synagogue to synagogue, a gabbai's responsibilities will typically include ensuring that the religious services run smoothly.

The gabbai may be responsible for calling congregants up to the Torah. In some synagogues, the gabbai stands next to the Torah reader, holding a version of the text with vowels and trope markings (which are not present in the actual Torah scroll), following along in order to correct the reader if the reader makes an error. In other synagogues, these responsibilities are instead that of a sgan (סגן).

A gabbai might manage some of the financial affairs of the institution, such as collection of contributions and keeping financial records. The administrator of charitable funds might be called the gabbai tzedakah.

A gabbai's responsibilities might also include maintaining a Jewish cemetery.