The clan branched off from the Heishi-Dohi clan and it is said to have started when Tohira KOBAYAKAWA, the son of Sanehira DOI, who was serving Yoritomo MINAMOTO, named himself as Kobayakawa from Kobayakawa of the north of the Dohi-go village (around Odawara City, Kanagawa Prefecture).
相模国を本拠地とする平氏土肥氏の分枝で、鎌倉時代初期、源頼朝に仕えた土肥実平の子・小早川遠平が土肥郷の北部・小早川（神奈川県小田原市付近）に拠って小早川の名字を称したのが始まりと伝わる。