Tadashi Yokouchi is a Japanese actor and voice actor born on July 1, 1941 in Dalian, Manchuria. He graduated from high school in Kitakyushu, Fukuoka Prefecture, Japan. He was a member of the 13th group of actors and actresses trained at the Haiyu-za acting school.

Yokouchi began his career in the late 1960s. NHK selected him for the lead role in the morning drama series Tabiji, which aired from 1967 to 1968. His career includes extensive work in live-action television, particularly in jidaigeki period dramas. He portrayed Atsumi Kakunoshin in the long-running series Mito Kōmon on TBS from 1969 to 1978, and he originated the role of historical figure Ōoka Tadasuke in Abarenbo Shogun on TV Asahi, playing the character from 1978 to 1997. He also appeared in the taiga dramas Shishi no Jidai (1980) and as Shimizu Yoshimasa in Fūrin Kazan (2007).

As a voice actor, his anime roles include Moog the Rock Giant in the Toei Animation film Hols: Prince of the Sun (1968), the Blonde Lensman in Lensman: Secret of The Lens (1984), and Oda Nobunaga in the 1986 animated film Toki no Tabibito -Time Stranger-. In the field of dubbing foreign productions, he is known as the Japanese voice of Carl Sagan for the series Cosmos: A Personal Voyage. He also provided the dub voice for J.R. Ewing, played by Larry Hagman, in the Japanese broadcast of the American television series Dallas.