Killock is the first signing by the NPL North West club's boss Wayne Benn ahead of the 2021/22 season.
Former Leeds United and Stockport Country defender Killock played for Boro in the 2018/19 campaign, joining from Stalybridge Celtic.
Killock said: “I’ve heard a lot of good things about the club, especially the fanbase who clearly follow the club in large numbers. I cannot wait to get started and play my part in seeing the club pushing forwards on the field."
Benn added: "He is just the calibre of player I am looking to bring to Ingfield. At 26, he has a lot of experience, and at a higher level, I expect Ross will be a leader both on and off the field.”
Another former Boro defender has committed to extend their stay with one of Athletic's rivals, Dom Roma signing for the 2021-22 season with Basford United in the NPL Premier Division.
Another of Boro's NPL Premier rivals, Buxton, have allowed former Scarborough keeper Tommy Taylor to leave the club at the end of his contract.