Upon commissioning, I-373 was attached formally to the Yokosuka Naval District and was assigned to Submarine Squadron 11 for workups. On 16 June 1945, she departed Yokosuka bound for Sasebo. Arriving at Sasebo on 17 June 1945, she began conversion into a tanker submarine capable of carrying 150 metric tons of aviation gasoline in addition to other cargo. On 20 June 1945, she was reassigned to Submarine Division 15 in the 6th Fleet.
