The former Bath star will wear the number 12 jersey for the first time in Gloucester colours after recovering from a fractured bone in his left wrist - an injury he picked up on England's summer tour to New Zealand.
Barkley will form a midfield partnership with skipper Mike Tindall, who shifts back to the outside centre position and means Matthew Watkins moves to the wing in place of James Simpson-Daniel, who is rested.
"This is has been a long time coming and I am really excited about the opportunity," Barkley said. "My rehabilitation has gone really well and now I just want to concentrate on contributing for my new club.
"Harlequins have made an excellent start to the season and we obviously appreciate their threat but after last week's performance and result we are confident we can re-create that again and built on it this weekend.
"I am just delighted to be involved in the side - it has been a long time coming."
Barkley joined Gloucester from Bath in February on a two-year contract and means Willie Walker continues at stand-off and Gareth Cooper at scrum-half.
Watkins' shift to the wing is the only change to the back three that also includes Lesley Vainikolo and full-back Olly Morgan.
Head coach Dean Ryan has made only one change to the forward pack that started at the Rec.
Alasdair Strokosch comes in at number six and Peter Buxton moves to the bench. Strokosch's other start - against Leicester on the opening weekend of the season - was on the openside flank.
However, Andy Hazell retains his number seven shirt and Luke Narraway, a try scorer in Gloucester's 21-17 win last Saturday, continues at number eight. Will James starts his third successive game at lock and will partner Alex Brown.
The front row of Nick Wood, Olivier Azam and Carlos Nieto also remain intact.
"Harlequins have been playing at a high tempo and have won well twice," said Gloucester's head coach Dean Ryan. "They are attack minded and are very dangerous when play breaks up - but I think we have taken on our game following the defeat against Leicester and have looked at things pretty objectively.
"We are a pretty free-flowing side but you can be challenged by playing too much rugby and we have to manage that. Last Saturday against Bath was much more of an illustration of what we have been working on.
"There will be consistency in selection in some areas but there will also be changes where we believe they are appropriate."
On the bench, Andy Titterrell is partnered by prop Alasdair Dickinson - who comes on in place of Dave Young - Marco Bortolami and Peter Buxton.
The three-quarter replacements are the same as last Saturday - scrum-half Rory Lawson, stand-off Ryan Lamb and winger Mark Foster.
There are still tickets available for tomorrow's game and they can be purchased from the ticket office hotline on 0871 871 8781 (option 2) online at www.gloucesterrugby.co.uk or from the ticket office on the day of the game.
GLOUCESTER (v Harlequins, tomorrow, 3pm): Olly Morgan; Matthew Watkins, Mike Tindall (capt), Olly Barkley, Lesley Vainikolo; Willie Walker, Gareth Cooper; Nick Wood, Olivier Azam, Carlos Nieto, Will James, Alex Brown, Alasdair Strokosch, Andy Hazell, Luke Narraway. Reps: Andy Titterrell, Alasdair Dickinson, Marco Bortolami, Peter Buxton, Rory Lawson, Ryan Lamb, Mark Foster.
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