Riku Sanjō, born October 3, 1964, in Oita Prefecture and raised in Tokyo's Shinagawa area, is a Japanese manga writer and screenwriter. A graduate of Meiji University's Department of Japanese Literature, Sanjō began his career writing for hobby magazines and OVA scripts, making his anime screenwriting debut with MD Geist in 1986. His professional manga debut as an original creator came with Skybomber Chokusen in 1987.

Sanjō is best known as the original creator of the manga Dragon Quest: The Adventure of Dai, which he wrote with illustrator Koji Inada. Serialized in Weekly Shōnen Jump from 1989 to 1996, the series became one of the best-selling manga in history, with over 50 million copies in circulation. The story follows a young orphan boy named Dai who dreams of becoming a hero after the defeat of the Demon King Hadlar, only to find himself confronting the resurrected demon lord and his master, the Great Demon King Vearn. Sanjō has continued to expand this universe as the original creator for subsequent manga entries, including the prequel Dragon Quest: Dai no Daibōken: Yūsha Avan to Gokuen no Maō, which began in 2020 and centers on the early adventures of Dai's mentor Avan, and the spin-off Dragon Quest: Dai no Daibōken Xross Blade, which ran from 2020 to 2024 and ties into the Xross Blade arcade and trading card game. Both works are credited to Sanjō as the original creator, with Yūsaku Shibata illustrating the prequel and Yoshikazu Amami illustrating Xross Blade.

Beyond his Dragon Quest work, Sanjō has demonstrated significant range across both manga and tokusatsu television. He co-created Beet the Vandel Buster with illustrator Koji Inada in 2002, a fantasy adventure series that has sold over four million copies. In 2017, he created Fuuto PI, a manga sequel to the live-action Kamen Rider W series. As a screenwriter, Sanjō has served as head writer for several Kamen Rider and Super Sentai series, including Kamen Rider W, Kamen Rider Fourze, Zyuden Sentai Kyoryuger, and Kamen Rider Drive, and he wrote every episode of Kyoryuger's 48-episode run, an unusual feat for the franchise.

Sanjō's artistic identity is characterized by a logical, structured approach to storytelling, which he has likened to mathematical problem-solving. His works frequently explore themes of heroism, mentorship, and the importance of understanding both allies and adversaries. He has emphasized the necessity of finding a personal challenge in each project, approaching popular franchises with the goal of delivering meaningful results that honor the original material while pursuing a unique vision suited to the medium. His scripts often reflect a collaborative spirit, as he makes a point of understanding the individual strengths of his creative partners, including voice actors and production staff, to integrate their qualities into his writing. Sanjō has also worked under several pen names including Ryu Sagawa and Rokurō Sano, though he produces his major works under his primary name.