SM UB-125 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 18 May 1918 as SM UB-125. UB-125 was surrendered 20 November 1918 in accordance with the requirements of the Armistice with Germany. Handed over to Japan, she served as O 6 in the Imperial Japanese Navy until 1921 when she was broken up in Kure.