Sadayasu was the son of Yasumune UTSUNOMIYA, the younger brother of Sadatsuna UTSUNOMIYA, and the sixth head of the Shimotsuke-Utsunomiya clan (the eighth counting from FUJIWARA no Soen), and went to Chikugo Province in Kyushu.
下野宇都宮氏第6代（藤原宗円から数えると第8代）の宇都宮貞綱の弟で九州の筑後国に下った宇都宮泰宗の子。