Sanseidō Co., Ltd. (株式会社三省堂, Kabushiki-gaisha Sanseidō) is a Japanese publishing company known for publishing dictionaries and textbooks.

The headquarters is situated in the area between Suidōbashi Station and Kanda River, at a location previously used as a warehouse for the company's own printing plant.

The company was founded in 1881 by Tadakazu Kamei [ja], a member of the old Hatamoto family, as the secondhand bookshop Sanseidō Bookstore Ltd. [ja]. It entered the publishing business in 1884.

From the outset, the company focused its business on academic fields, publishing dictionaries, encyclopedias, textbooks, and for a period, maps, as well as selling educational materials. While its rival in the field of dictionary publishing at the time, Fuzanbō [ja], focused on large, specialist-oriented dictionaries, Sanseidō concentrated on smaller but more practical dictionaries. For this purpose, the company opened its own printing plant in 1889.

On October 18, 1912, two months after the publication of Nihon Hyakka Daijiten's sixth edition, the company went bankrupt. This came as a surprise to the company's employees and general public considering the prevalence of the company's dictionaries across the country. In 1915, at the suggestion of Tadakazu's son, Torao, who was a university student at the time and had a background in economics and accounting, the publishing business and bookstore would be separated. He would later go on to join the company after graduation, later becoming president in July 1930.

In the late 1970s, the company's publishing business was in a slump, and soon the company went bankrupt before reviving again in 1881 with the printing division spinning off as SANSEIDO Printing Co., Ltd. [ja].

Shin Meikai kokugo jiten is the highest-selling Japanese-language dictionary in Japan, and is well known for its unique and straightforward definitions and examples. It was compiled almost single-handedly by chief editor Tadao Yamada [ja], and his ideas are strongly reflected in the dictionary. Yamada had previously worked alongside Hidetoshi Kenbou [ja] on its predecessor, Meikai kokugo jiten [ja]. Kenbou would later become editor of Sanseido Kokugo Jiten.

While working together on Meikai kokugo jiten, the two conflicted in the direction they wished the dictionary to go, resulting in the creation of the two new dictionaries following. The philosophy behind Shin Meikai kokugo jiten would prefer to retain older words, even if had begun to fall out of common use, and its text didn't hesitate to give lengthy explanations. Sanseido kokugo jiten on the other hand's philosophy desired to keep explanations concise, hoping to include as many new words as possible.