Toshio Saeki (佐伯 俊雄, Saeki Toshio) is a fictional character from the Ju-on franchise, created by Takashi Shimizu. He was played by several child actors, including Daiki Sawada in the short movie 4444444444, Ryōta Koyama in the original two V-Cinema movies, Yuya Ozeki in the theatrical movies and The Grudge, Ohga Tanaka in The Grudge 2, Shimba Tsuchiya in The Grudge 3, Shūsei Uto in Ju-on: White Ghost and Ju-on: Black Ghost, Kai Kobayashi in the 2014 Japanese reboot Ju-on: The Beginning of the End, and Garrett Masuda in the parody Scary Movie 4. He is characterized by his pitch-black eyes, pale skin, and the haunting meowing sounds he makes.

He can be considered a prominent character in the series, second to Kayako, his mother. His first appearance was in Takashi Shimizu's short movie 4444444444 and his haunting goes all through the franchise series in a house in Nerima, Japan. He is one of the enactors of the curse, which states that when someone dies amidst rage and intense wrath, those feelings remain at the death site and kill whoever it touches.

His next appearance was in The Grudge 3, played by Shimba Tsuchiya. He also has a guest appearance in Ju-on: White Ghost and Ju-on: Black Ghost respectively, the 2009 Japanese sequels and both are played by Shūsei Uto.

Toshio has his first major appearance in the 2014 reboot Ju-on: The Beginning of the End and its sequel Ju-on: The Final Curse, where unlike the previous films, he is the main antagonist and the ultimate cause behind the curse.

• In the American remake, Takeo kills Toshio by drowning him in the bathtub. Unlike in the Japanese original, where Takeo kills the cat with a utility knife, in the American remake he also drowns him in the bathtub. He puts Toshio's body in the closet and seals it with duct tape.
• In the Ju-on novel, Toshio hides in the attic from his father. Instead of coming after him, Takeo slams the cupboard door shut, leaving Toshio to starve to death. However, whilst in the attic, Toshio encounters the body (and the ghost) of his mother, so it is unknown if he did indeed starve, or if Kayako claimed him. Also in the novel, Toshio buries his own cat in the dirt, after both of their deaths.
• In the Ju-on films, Toshio is six years old at the time of his death. In the Grudge films, he was either seven or eight years old (the online newspaper article that Karen reads states Toshio's age as seven, but a link to another online article on the search engine she uses states Toshio's age as eight).
• In the Ju-on films, Takeo killed Toshio because he wrongly suspected that he was not his son, and was instead the product of an affair between Kayako and Kobayashi. The Ju-on novelization expands on this, by detailing that Takeo and Kayako were unable to get pregnant again no matter what they tried, and at his doctor's appointment he was informed that he had a low sperm count, fueling his suspicion that Toshio wasn't his son. In the American films, Takeo had to kill Toshio because he walked in on him killing Kayako. It's unlikely that he suspected that Toshio wasn't his son in the American films, as Peter Kirk, Kobayashi's American counterpart, is a white man.
• In the Ju-on: The Beginning of the End reboot, there are two Toshio identified: Toshio Yamaga, who resided in the house nineteen years before the main timeline, and Toshio Saeki, who was reborn in Kayako's womb ten years before the main timeline. Toshio is also older than the previous continuities, being at least 9 years old at his death. While the backstory is completely changed as mentioned above, Toshio's pet cat, Mar, is still retained.