A Gabbai (Hebrew: גבאי‎) (or sometimes: Shamash שמש) is a person who assists in the running of a synagogue and ensures that the needs are met, for example the Jewish prayer services run smoothly, or an assistant to a rabbi (particularly the secretary or personal assistant to a Hassidic Rebbe). A gabbai's obligations might also include maintaining a Jewish cemetery.

In many synagogues the gabbai is not a permanent job like the one described above but rather a role in the Torah service. The gabbai is 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 trop markings (which are not present in the actual Torah scroll), following along in order to correct the reader if he makes an error (e.g., mispronounces a word, or skips a word). In others, this is separated out into the role of sgan סגן.

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

David Dario Gabbai (born 1922) is a Greek Sephardi Jew and Holocaust survivor, notable for his role as a member of the Sonderkommando at Auschwitz. He was deported to the camp in March 1944 and put to work in one of the crematoria at Birkenau, where he was forced to assist in the burning of the hundreds of thousands of Hungarian Jews that arrived during the spring and summer of that year.

Gabbai remained at Auschwitz until its evacuation in January 1945. He was liberated from Ebensee concentration camp in Austria by the United States Army, and has publicly spoken about what he witnessed and experienced during the Holocaust.

Gabbai was born in Thessaloniki to a Greek mother and an Italian father, and was educated in Italian schools in Greece. At the age of 21 or 22 years old, Gabbai and his entire family were detained by the Nazis on March 24, 1944, and on April 1 they were sent to Auschwitz in cattle wagons. Ten days later, this transport arrived at the Judenrampe outside Auschwitz-Birkenau, where it faced the selection process. With the exception of Gabbai himself, his brother, and his two cousins, the entire family were selected for extermination and gassed the same day. Gabbai watched his parents being loaded onto the trucks that would take them to the crematoria and gas chambers.