Mitsui (written: 三井 lit. "three wells") is a Japanese surname. Notable people with the surname include:
Aika Mitsui is a member of Hello! Project and a former member of the J-pop group Morning Musume, joining as a member of the eighth generation from the audition in Japan during December 2006. Mitsui's audition process was documented during the late 2006 episodes of Hello! Morning. She performed Ayumi Hamasaki's "Blue Bird" in round one of her audition and Morning Musume's "Furusato" and "Osaka Koi no Uta" in rounds two and three. She graduated from Morning Musume on May 18, 2012. She retired from the entertainment industry to pursue culinary studies in New Zealand on November 3, 2018.
Hideki Mitsui is a Japanese screenwriter. He has composed the script and series composition for numerous anime series.

Kōji Mitsui was a Japanese movie, TV, and stage actor. He appeared in more than 150 films from 1925 to 1975, including 29 of Kinema Junpo’s annual Top-10 winners and three of its Top-10 best Japanese films of all time.
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Sumitomo Mitsui Financial Group, Inc. is a Japanese bank holding / financial services company established by Sumitomo Mitsui Banking Corporation, the second largest bank in Japan by market value.
Sumitomo Mitsui Banking Corporation is a Japanese multinational banking and financial services company headquartered in Yurakucho, Chiyoda, Tokyo, Japan. It is a wholly owned subsidiary of Sumitomo Mitsui Financial Group. SMBC is the second largest bank in Japan by assets.
Mitsui Sumitomo Insurance Group Holdings, Inc. is a Japanese insurance holding company headquartered in Tokyo, Japan.
Mitsui Fudosan Co., Ltd. is a major real estate developer in Japan. Mitsui Fudosan is one of the core companies of Mitsui Group.
Mitsui Sumitomo Visa Taiheiyo Masters is an annual golf tournament on the Japan Golf Tour. Its previous name include Taiheiyo Club Masters, Toshiba Taiheiyo Masters,Visa Taiheiyo Club Masters, and Sumitomo Visa Taiheiyo Masters. It is played in November and in recent years at the Taiheiyo Club, Gotenba Course, Gotenba, Shizuoka and attracts some of the leading international golfers. It was founded in 1972. Its title sponsors are Sumitomo Mitsui Banking Corporation and Visa.
Miike coal mine , also known as the Mitsui Miike Coal Mine , was the largest coal mine in Japan, located in the area of Ōmuta, Fukuoka and Arao, Kumamoto, Japan.

Wakio Mitsui is a Japanese politician of the Democratic Party of Japan, a member of the House of Representatives in the Diet. A native of Sapporo, Hokkaido and graduate of Showa Pharmaceutical University, he was elected to the House of Representatives for the first time in 2000 after an unsuccessful run in 1996.
Mitsui Chemicals is a Japanese chemicals company listed on the Nikkei with business interests in Japan, Europe, China, Southeast Asia and the USA. It is a part of the Mitsui conglomerate. The company mainly deals in performance materials, petro and basic chemicals and functional polymeric materials.
The Mitsui Memorial Museum is an art museum in Tokyo's Nihonbashi district. It is located within the Mitsui Main Building, an Important Cultural Property as designated by the Japanese government.
The Nishitetsu Amagi Line is a Japanese railway line operated by the private railway operator Nishi-Nippon Railroad (Nishitetsu), which connects Kurume and Asakura in Fukuoka Prefecture.
The Nihonbashi Mitsui Tower (日本橋三井タワー) is a skyscraper located in Chuo, Tokyo, Japan. The 192-metre, 34-storey building is primarily used for office space with the upper floors occupied by a hotel. Its construction was completed in 2005. It is connected by an underground concourse to the Tokyo Metro Mitsukoshimae Station.
Mitsui Engineering & Shipbuilding is a Japanese company. It is listed on the Nikkei 225.
Koji Mitsui is a former pitcher in Nippon Professional Baseball. Mitsui pitched for the Seibu / Saitama Seibu Lions from 2001 through 2009.
Sumitomo Mitsui Trust Holdings, Inc. , formerly Chuo Mitsui Trust Holdings, Inc., is a Japanese financial holding company headquartered in Chiyoda, Tokyo. It provides an assortment of financial solutions and financial products to retail and wholesale customers, with a focus on asset management, financial brokerage and real estate services.
Yōko Mitsui was a Japanese poet.
Risako Mitsui is a Japanese synchronized swimmer.
Sumitomo Mitsui Construction Co., Ltd. is a Japanese general construction company based in the Chuo ward of Tokyo. The company traces it routes back to 1887, when Nishimoto-Gumi, the predecessor of Mitsui Construction, was established. This company was incorporated in 1941. The current company was formed in 2003 when Mitsui Construction and Sumitomo Construction agreed to merge. The company is part of the Mitsui Group and Sumitomo Group.
Tomomi Miyamoto is a former Japanese football player. She played for Japan national team.
Mitsui Hachirōemon is the name of the head of the Sōryō branch of the Mitsui family.