Kōsei Yagi is a Japanese voice actor whose career includes a range of roles in anime television series, films, and original video animations, with much of his work concentrated from the 1960s through the early 2000s.

One of his notable early roles is the character Professor in the 1966 science fiction adventure series Rainbow Sentai Robin, where he voiced a highly intelligent robot who was a member of the main cast. In the 1997 animated film The Dog of Flanders, Yagi voiced the role of Nello's Grandfather (Jehan), the kind and caring guardian of the young protagonist. His other significant roles from the 1990s include Youichirou Techno in the 1997 television series Don't Leave Me Alone, Daisy, and the role of Ayushi in the 2000 OVA The Prince and the White Horse. His film credits from this period also include the role of Elran in Mobile Suit Gundam II: Soldiers of Sorrow (1981) and Kouichi Nakamura in Blue Submarine No. 6 (1998 to 2000). He also contributed to the long-running series Detective Conan as the character Kuranosuke Take.