The 19-year-old made four appearances off the bench for the Easter Road side last term and featured in pre-season, impressing against Arsenal and scoring against Raith Rovers, and has extended his stay with the Capital club until the summer of 2024.
Head coach Jack Ross spoke highly of his impact during the summer and Bradley will now turn out for David Hopkin’s side in the second tier after agreeing a season-long loan move, although Hibs have a recall option.
Sporting Director Graeme Mathie said: “Firstly, we are delighted that Steven has signed an extended contract. He’s a player we rate highly and are very pleased to have him as a Hibernian player for the next three years.
“He has benefited from being part of Jack Ross’ first team group for the last season and will continue to develop under the guidance of David Hopkin and Jim Duffy at Ayr United.
“The loan is a good opportunity for Steven to play regular football and his challenge will be to gain regular match minutes in the Scottish Championship, which will be a very competitive league this year.
“We wish him every success with his loan and look forward to seeing him back at HTC in the not-too-distant future.”
Hopkin added: “We are delighted to be able to get Steven in. We tried earlier in the window however at that time Jack was keen to hold onto him.
"He has been involved in their first-team set up for some time now and has been showing up extremely well.
"Steven is an exciting player who will excite the fans and after the issues we have had with Covid we needed to strengthen the squad."
Bradley will go straight into the Somerset Park side’s squad for Tuesday night’s league match against Raith Rovers, with Hibs’ deadline-day signing Dylan Tait expected to feature for the visitors.
Glasgow-born Bradley began his career with Rangers, playing alongside Scotland internationalist Nathan Patterson but was released and returned to boys’ club football before Queen’s Park picked him up and handed him his debut at the tender age of 16.
He joined Hibs after impressing during an Under-18s match.