I-52 (伊号第五二潜水艦 (伊52), I Gō Dai Gojūni Sensuikan (I Gojūni); I-52 submarine (I-52)), code-named Momi (樅; "fir tree") was a Type C2 cargo submarine of the Imperial Japanese Navy used during World War II for a secret mission to Lorient, France, then occupied by Germany, during which she was sunk.

She is also known as Japan's "Golden Submarine", because she was carrying a cargo of gold to Germany as payment for matériel and technology. There has been speculation that a peace proposal to the Allies was contained on board the I-52 as well, but this is unlikely on two counts: there is no evidence that the Japanese government was interested in peace proposals or negotiated settlements at that stage in the war; and the Japanese kept an open dialogue with their diplomatic attachés via radio and diplomatic voucher through Russia, and had no need for long and uncertain transfer via a submarine bound for a Nazi-controlled area of western Europe.

It is believed that 800 kg (1,760-lbs) of uranium oxide awaited I-52 for her return voyage at Lorient according to Ultra decrypts. It has been speculated that this was for the Japanese to develop a radiological weapon (a so-called "dirty bomb") for use against the United States. (The amount of unenriched uranium oxide would not have been enough to create an atomic bomb, though if used in a nuclear reactor it could have created poisonous fission products).

She was also to be fitted with a snorkel device at Lorient. In addition, 35 to 40 tons of secret documents, drawings, and strategic cargo awaited I-52's return trip to Japan: T-5 acoustic torpedoes, a Jumo 213-A motor used on the long-nosed Focke-Wulf Fw 190D fighter, radar equipment, vacuum tubes, ball bearings, bombsights, chemicals, alloy steel, and optical glass.

This class of submarines was designed and built by Mitsubishi Corporation, between 1943 and 1944, as cargo carriers. They were quite long and carried a crew of up to 94. They also had a long cruising range at a speed of 12 knots (22 km/h). The Japanese constructed only three of these submarines during World War II (I-52, I-53 and I-55), although twenty were planned. They were the largest submarines ever built at that time, before the enormous Sentoku subs were built, and were known as the most advanced Japanese submarines of their time.

The keel of I-52 was laid on 18 March 1942, and she was commissioned on 28 December 1943 into the 11th Submarine Squadron, Kure Naval District, with Commander Kameo Uno as the commanding officer. In January 1944, Admiral Mineichi Koga, the commander of the Combined Fleet, selected I-52 for a special mission to Germany.

Yanagi missions were enabled under the Axis Powers' Tripartite Pact to provide for an exchange of strategic materials and manufactured goods between Germany, Italy and Japan. Initially, cargo ships made the exchanges, but when that was no longer possible submarines were used.

Only five other submarines had attempted this intercontinental voyage during World War II: I-30 (April 1942), I-8 (June 1943), I-34 (October 1943), I-29 (November 1943), and German submarine U-511 (August 1943). Of these, I-30 was sunk by a mine, I-34 by the British submarine HMS Taurus, and I-29 by the United States submarine Sawfish (assisted by Ultra intelligence).