It becomes Killock’s second spell at the Lookers Vauxhall Stadium, having spent a season on loan back in the 2013/14 season.
The 21-year-old defender has had loan spells at Alfreton Town and FC Halifax Town in the National League whilst at Championship side Leeds United.
Killock said: "As soon as Jon McCarthy rung me, I knew I wanted to sign right away. My time here was probably my best in football, so it's great to be back."
Ross becomes Blues boss McCarthy’s third summer signing after fellow defender Blaine Hudson signed on the dotted line last Tuesday, and Elliott Durrell put pen-to-paper earlier.