Morio Kasai (葛西 森夫; September 29, 1922 – December 8, 2008) was a Japanese surgeon who had a strong interest in pediatric surgery.

While Kasai went into practice at a time when pediatric surgery was not an established subspecialty, much of his clinical and research work was related to the surgical care of children.

Kasai's early career interests included postoperative fluid and electrolyte management as well as the care of infants and children with peritonitis.

[1] In 1959–60, Kasai spent nine months in the United States completing a pediatric surgical fellowship under C. Everett Koop at Children's Hospital of Philadelphia (CHOP).

[2] Kasai and a colleague, Sozo Suzuki, worked together in the 1950s to devise a surgery to treat babies born with biliary atresia, a typically fatal condition of the gastrointestinal tract.

One day, Kasai encountered significant bleeding near the portion of the liver known as the porta hepatis while trying to dissect an infant's ductules.

[1] He also devised a surgery to correct Hirschsprung's disease (rectal myotomy with colectomy); he noted that it involved less surgical exploration in the pelvic area and he felt that this would reduce the loss of sensation in the rectum in comparison to the common procedure at that time.

[7] In the 1970s, Kasai came to the United States again to work with Koop and his colleague Louise Schnaufer at CHOP, where they established a specialized surgical program for biliary atresia.