The Whites have a buy-out option to sign the Manchester City winger, 22, who featured 42 times last season for the Whites, permanently next summer.
Meanwhile, Brighton defender Ben White has completed his expected move to Elland Road on a season-long loan, with Leeds also aiming to bring in Wolves winger Helder Costa on loan following the completion of their first incoming summer deals so far.
On returning to Leeds, Harrison said: “It is exciting to be back, the end of last year was disappointing with how it ended, so this year myself and the lads are looking to redeem ourselves and build on last year.
“I think most people felt that we should have gone up last season, but we are all looking to get back at it now and hopefully we can get the promotion this time around.”
Meanwhile, White, 21, is confident that working with Leeds head coach Marcelo Bielsa will prove beneficial for his burgeoning career.
The Brighton academy product spent time out on loan at Newport County during the 2017-18 campaign, before joining Peterborough United in January where he made 16 appearances for the League One side.
On heading to Leeds, White said: “When I heard of the opportunity to come, I was just so excited to get it over the line.
“I have heard loads about Marcelo Bielsa, he is an amazing manager, especially with young players. I am sure he is going to really improve me, which is what I am looking to do this season.”
Leeds have also completed the signing of highly-rated young Carlisle United winger Liam McCarron, 18, for an undisclosed fee, with the Appleby-based teenager signing a three-year deal.
The fee is reportedly in the region of £250,000 to £300,000 for McCarron, who will begin life in Leeds’s under-23 squad.
Meanwhile, Huddersfield Town first-team coach Mark Hudson has signed a new-contract with the club.
Rotherham United remain interested in Ross County midfielder Jamie Lindsay, subject of a failed £150,000 bid last month.
The Millers, who are not expected to revisit their interest in Southend’s Dru Yearwood after seeing an offer rejected, are in the market for a number of players, including a goalkeeper and a wide-sided player.
Blackpool defender Curtis Tilt remains a target, despite the Millers seeing one offer rebuffed.
On Lindsay, manager Paul Warne said: “It is true that we have had a bid turned down. They want more money.
“That is the next step for us, whether we up our offer or take a different option.”
Former Doncaster Rovers captain Tommy Rowe has signed a two-year deal with Bristol City, while young Middlesbrough defender Enes Mahmutovic has joined Dutch side MVV Maastricht on a season-long loan.