Australia's footballing world was shocked when Sykes announced her retirement from the professional game at the end of the 2017-18 season at only 26.

Her decision came after 10 years in the then-W-League took a toll on her desire to play the game.

Now United's most capped player will don the green again come December 3.

The 29-year-old rediscovered her love for the game in the ACT's NPLW this season with Canberra Olympic, under the coaching direction of her twin-sister, former United player Nicole Begg.

The side finished fifth, and Sykes took out player of the year and finished runner-up in the competition's golden boot with 21 goals to her name.

"I found my love for football again," she said.

"The players and coaches made it fun, and reminded me how much joy can be found on a football field. I'm hoping to get similar enjoyment from playing for Canberra United again, it's always been my experience here and the team have made me feel welcome.

"I missed the feeling of competing every day with your teammates and for spots on the team. I love the challenge that this comeback presents. Getting match fit and back up to the standards that the A-League Women demands is my driving force at the moment."

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Sykes made her debut for United at 16, before accumulating 113 appearances for the club across 11 seasons, slotting away 41 goals in the process, and securing three premierships and two championships.

The attacker is hoping she will be able to link up with Michelle Heyman again up front and build on the side's success last season, which saw them qualify for finals for the first time since 2016-17.

She will also link up with Orlando Pride import Chelsee Washington in the forward line.

"It's always a privilege to wear the Canberra United jersey and play for this club," Sykes said.

"I'm super excited by the opportunity from Vicki and the club to have me play for them. I hope to make an impact for the team and add some more firepower to our already talented attacking stocks."

It's not the first time United head coach Vicki Linton has taken a gamble on former ALW stalwarts, after Heyman took a year off and returned last season.

That move paid off for Linton, and she believes this one will too.

"Ash is a quality player with so much experience and has been a massive part of this club from day one," Linton said.

"I had the chance to work with her in my time with the Matildas and to be able to reunite with her here at club level is such a nice opportunity.

"Already she has shown her ability and intelligence as a player and she will no doubt have a positive impact for this team with her performances in games but also with her mentality and the culture she can bring in training.

"I am really glad she has this chance to come back and compete at a high level again and enjoy her football."

Sykes' had been training with the side for the majority of pre-season, before her signing was confirmed on Wednesday. It comes just in time for United's first game against Melbourne City at Viking Park next week.