At 08:20 on 30 August 1945, "I-400" got underway in company with "I-14" and "Proteus" bound for Tokyo Bay, where they anchored at 14:58 the same day. The United States flag was raised aboard her at 05:00 on 31 August 1945, and at 14:45 "I-400", "I-14", and "Proteus" began a move to a new anchorage within the breakwater at the submarine base at Yokosuka, where "Proteus" was anchored by 16:50. Twelve U.S. Navy submarines chosen to represent the U.S. Navy Submarine Force at the Japanese surrender ceremony in Tokyo Bay also were anchored there, and "I-401" arrived on 31 August as well. During the surrender ceremony on 2 September 1945, the Commander, Submarine Force, U.S. Pacific Fleet (COMSUBPAC), Vice Admiral Charles A. Lockwood, ordered his personal flag hoisted aboard "I-400" and "I-401".