SM UB-129 was a German Type UB III submarine or U-boat in the German Imperial Navy () during World War I. She was commissioned into the German Imperial Navy on 11 June 1918 as SM UB-129.
UB-129 was lost 31 October 1918 in Fiume () after the surrender of Austria-Hungary.
She was built by AG Weser of Bremen and following just under a year of construction, launched at Bremen on 11 May 1918. UB-129 was commissioned later the same year under the command of Kptlt. Karl Neumann. Like all Type UB III submarines, UB-129 carried 10 torpedoes and was armed with a deck gun. UB-129 would carry a crew of up to 3 officer and 31 men and had a cruising range of . UB-129 had a displacement of while surfaced and when submerged. Her engines enabled her to travel at when surfaced and when submerged.