Red Boy (traditional Chinese: 紅孩兒; simplified Chinese: 红孩儿; pinyin: Hóng Hái-er; Wade–Giles: Hung² Hai²-êrh; Jyutping: Hung4 Haai4ji4; Vietnamese: Hồng Hài Nhi, Japanese: Kōgaiji) is a character featured in the 16th-century novel Journey to the West. Red Boy was also known as the Boy Sage Great King (聖嬰大王; Shèngyīng Dàwáng) and "Bull Boy Sage" (牛聖嬰; Niú Shèngyīng). Through this, Red Boy had developed True Samādhi Fire (三昧真火; Sānmèi-zhēnhuǒ), which enables him to shoot fire inextinguishable by water and smoke from his eyes, nostrils, and mouth. Samādhi is the Sanskrit word for concentration. His classical appearance is Sudhana also known as Shancai.

Red Boy (紅孩兒) is the ferocious son of the Bull Demon King and Princess Iron Fan (a Rakshasa). Upon hearing about Tang Sanzang's arrival at his mountain, he attempted to capture the monk so he and his father could eat the latter's flesh and extend their lifespans. Although a young child, he is fiercely independent and assigned to a land away from his parents, even reigning over lesser demons and local deities with his mighty power.

Red Boy first meets Sun Wukong, Tang, Zhu Bajie, and Sha Wujing as a red cloud emerging from Mount Hao, which alerts them to his presence. He then turned into a ball of fire and disappeared. Red Boy tries to lure the travelers by disguising himself as an innocent boy tied to a tree; they kindly untie him, and Sun Wukong (who knows that he's a demon) has to carry him along the way until he prepares to kill him afterward. But Red Boy escaped his spirit out of his body just in time as Wukong violently destroyed it. Enraged that Wukong tried to kill him, Red Boy conjures a powerful whirlwind around him and kidnaps Tang Sanzang in the chaos.

Red Boy brought him to his Fire-Wind Cave near the Withered Pine Ravine in Mount Hao. He prepared to cook Tang Sanzang but was alerted by the lesser demons that Sun Wukong and Zhu Bajie were demanding his master back outside the cave. Red Boy ordered the lesser demons to push five carts outside the cave in the order of the Five Elements.

He did not believe Wukong's statement that his father (the Bull Demon King) was Sun Wukong's sworn brother, which technically made Wukong a relative of his.

Red Boy and Wukong fought for 20 rounds. When Zhu Bajie joined the fight, Red Boy stood in the middle of one of the five carts and punched himself twice in the nose. He said a spell and breathed out fire with smoke from his nostrils. Fire blew out of the five carts and spread everywhere, blotting the skies.

Wukong cast a fire resistance spell and chased after Red Boy but could not find him in the flames, so he jumped out of the fire. Red Boy, who had returned to his cave, thought he had defeated Wukong.

Sun Wukong at first asks for some rain from the Eastern Dragon King to counter Red Boy's Samadhi Fire, to no avail (the Dragon King's rain can only extinguish normal fires, but Red Boy's fire is inextinguishable from normal efforts to quench fire), with the rain in fact intensifying the flames. Caught directly by the flames, Sun Wukong is ultimately almost burned to death, and he finally resorts to asking for the help of the Guanyin.