Dentsu was originally established as Japan Advertising Ltd. (日本広告株式会社, Nihon Kōkoku Kabushiki-gaisha) and Telegraphic Service Co. (電報通信社, Denpō Tsūshin-sha) by Hoshiro Mitsunaga [ja].

[8] Dentsu's sales are more than double its nearest competitor, Hakuhodo or ADK, in the Japanese market, thanks to the company's origins as a media representative during the early part of the 20th century, producing the first newspaper advertisements as well as the first television commercials in Japan.

On 12 July 2012, Dentsu agreed to acquire British-based Aegis Group plc in a cash deal worth $4.9 billion.

In the same month, Dentsu Ventures Global Fund I was launched with a total capital of five billion yen.

[15] In May 2021, Dentsu Ventures Global Fund II was launched with a total capital of ten billion yen.

[16] Dentsu Ventures investments include Cheddar, Clear Labs, Exo, GRAIL, Overtime, OpenWeb, Survios, Twist Bioscience, Tynker, and Upside Foods.

[18] In August 2023, it was announced Dentsu had acquired the Berlin-headquartered digital-first brand, communications and creative agency, RCKT.

[22] In December, Matsuri Takahashi, a University of Tokyo graduate and 24-year-old female employee of Dentsu, committed suicide.

[27] Over a half century of ongoing overworking was documented by The New York Times,[23] including training materials supporting a Dentsu president urging working "even if it kills you."

[28] Also in 2017, a former executive creative director of Dentsu Japan has resigned from his own company following allegations he sexually harassed a woman during his tenure at the advertising giant.

In February 2023, Japan's competition regulator filed criminal complaints against six firms, including Dentsu, over alleged bid rigging of contracts for the games.

[31] The corruption scandal has clouded the chance of landing the 2030 Winter Olympics by the city of Sapporo, an early favorite.

[32] According to one media report, Dentsu does business with almost every major institution in Japan, accounting for about 28 percent of the national advertising budget.