Isai Zelbst was born in 1911 in Verkhneudinsk. The Zelbst family probably originated from northeastern Belorussia. Isai's father Leontii Zelbst participated in the Russo-Japanese war of 1904-1905 and received Russian military awards. After this war, as a veteran, he did not return to his native region but settled in Verkhneudinsk, Transbaikalia, now Ulan-Ude, Buriatia). After finishing school and before his mobilization into the Red Navy in 1931, Isai worked as a mechanic at the Ulan-Ude railway junction.
In 1935 Zelbst graduated from the Dzerzhinskii Naval Engineering Higher School in Leningrad (now St. Petersburg) as an engineer-mechanic for submarines. He was assigned to the Soviet Black Sea Fleet and served as an officer on various medium-sized submarines (that were referred to in the Soviet nomenclature as Shch-class submarines- unofficial name Shchuka [lit. pike fish]). In 1940 Isai Zelbst was promoted to captain 3rd class. He began the war between the Soviet Union and Germany in June 1941 as a deputy commander of a medium size submarine; in September 1941 he rose to the post of the commander of a Shch-210, a torpedo submarine. His attack on the Vichy France's tanker Lé Progrès near the Bulgarian sea shore in October 1941 did not succeed (the torpedo exploded before reaching the ship. However, Zelbst did manage to free the submarine from the Romanian "sea hunters" that were bombing it with depth charges.
In March 1942 Shch-210 did not return to the naval base in Tuapse. According to a postwar investigation, the submarine was exploded by a Romanian mine four nautical miles from the Bulgarian shore, near the border with Romania. All the crew perished.
Isai Zelbst was posthumously awarded the Order of the Red Banner. An article on him was prepared for the Soviet Yiddish newspaper Eynikayt.
Another member of the Zelbst family from Ulan-Ude, Lieutenant Abram Zelbst, the commander of a mortar platoon of the 112th Rifle Division (1st Ukrainian Front), was awarded the Order of the Red Star and medals in 1944-1945.