Kaisei Town is located in the western part of Kanagawa Prefecture and belongs to Ashigarakami District.
The town's name originates from the "Kaisei School" established in 1882 (Meiji 15) in Enzawa Village. In 1901, it merged with Sakata Village and Yoshidajima Village to form "Kaisei Elementary School," which is now "Kaisei Municipal Kaisei Elementary School." The term "Kaisei" comes from a phrase in the I Ching (Book of Changes), specifically from the Xici (Appended Remarks): “To open up things and accomplish tasks.”
Kaisei Town is the smallest municipality in terms of area in Kanagawa Prefecture. After Shin Town in Tano District, Gunma Prefecture, merged into Takasaki City in 2006 (Heisei 18), Kaisei also became the smallest town by area in the eastern Japan region.