Watford have announced the signing of Italy international Marco Motta, subject to international clearance.
Mark Noble has signed a new deal with West Ham which will keep him at the club until the summer of 2020.
The 19-year-old, who left Tynecastle earlier this summer before the club entered administration, is Pat Fenlon's fourth signing of the summer.
Mullen has represented Scotland at U17 level and joins Owain Tudur Jones, Liam Craig and Ryan McGivern as Hibs' summer signings.
A statement on Hibernian's website said: "Hibernian can confirm the club has signed defender Fraser Mullen on a one-year contract.
"A former Scotland under-17 internationalist, right-back Mullen, 19, agreed the deal on Saturday morning and has become the club's fourth arrival during the summer transfer window."
Hibs will depart for a training camp in southern Europe on Monday afternoon following their 1-1 draw with Dumbarton in their first pre-season friendly on Saturday.