SM UB-125 was a German Type UB III submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) 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.
See main article: Type UB III submarine. She was built by AG Weser of Bremen and following just under a year of construction, launched at Bremen on 16 April 1918. UB-125 was commissioned later the same year under the command of Kptlt. Fritz Schubert. Like all Type UB III submarines, UB-125 carried 10 torpedoes and was armed with a 10.52NaN2 deck gun. UB-125 would carry a crew of up to 3 officer and 31 men and had a cruising range of 7280nmi. UB-125 had a displacement of 512lk=inNaNlk=in while surfaced and 643t when submerged. Her engines enabled her to travel at 13.9kn when surfaced and when submerged.
| Date | Name | Nationality | Tonnage[1] | Fate[2] | |
|---|---|---|---|---|---|
| 29 August 1918 | Atxeri Mendi |  Spain | 2,424 | Sunk | |
| 30 August 1918 | Onega |  United States | 3,636 | Sunk | |
| 1 September 1918 | Actor |  United Kingdom | 6,082 | Damaged | |
| 3 September 1918 | Brava |  Portugal | 3,184 | Sunk | |
| 3 September 1918 | Lake Owens | United States | 2,308 | Sunk | |
| 4 September 1918 | Bogstad |  Norway | 1,589 | Sunk | |
| 12 September 1918 | Skjold |  Denmark | 166 | Sunk |