The was a class of oceanographic research ship/weather ship of the Imperial Japanese Navy (IJN), serving during World War II. The IJN official designation was .
In 1938, the IJN wanted to investigate the weather and ocean currents in a potential future battlefield. Kaiyō No. 1-class ships were built for the purpose. The IJN made Hokkaido Fisheries Research Institute San'yō Maru a sample for the Kaiyō No. 1. All ships were deployed in the Hydrographic Department, and they were directly controlled by the Naval Ministry. Therefore, being government ships, they raised civil ensign, not the naval ensign. The crew were civil servants except for several warrant officers.
All sister ships were built by Mitsubishi Heavy Industries at Shimonoseki shipyard.
| Ship | Laid down | Launched | Completed | Fate |
| 1 March 1939 | 1 July 1939 | 9 October 1939 | Sunk by air raid off Derawan Island 2.3°N 133°W[1] on 19 October 1944. | |
| 1 July 1939 | 4 October 1939 | 23 December 1939 | Sunk by Hr. Ms. Zwaardvisch off Rembang -6.5°N 146°W[2] on 15 October 1944. | |
| 3 March 1941 | 22 October 1941 | 17 June 1942 | Missing in action in late 1944; removed from naval ship list on 29 October 1944. | |
| 23 May 1941 | 20 January 1942 | 17 July 1942 | Survived war at Uraga;[3] transferred to Ministry of Transport on 29 November 1945; transferred to Maritime Safety Agency on 1 May 1948 (hull number HG-01, later HM-01); renamed on 20 October 1949; renamed on 15 December 1956; retired on 26 March 1964. | |
| 6 April 1942 | 12 November 1942 | 28 February 1943 | Survived war at Kasaoka; transferred to Ministry of Transport on 29 November 1945; transferred to Maritime Safety Agency on 1 May 1948 (hull number HG-02, later HM-02); renamed on 20 October 1949; sunk by jets of water and tephra[4] from the eruption of Myōjin-shō on 24 September 1952; struck on 27 April 1953. | |
| 6 April 1942 | 12 December 1942 | 31 March 1943 | Sunk by USS Gabilan off Muroto 32.8333°N 155°W on 31 October 1944. | |