The 34-year old joins Canberra after a strong Euros campaign with England, notching more than 60 appearances for her country.
Bassett said she couldn't wait to start with United, who took out last season's premiership, before Melbourne City went on to claim the W-League championship.
"I'm so excited and I am really looking forward to meeting all the staff and players at this ambitious club," Bassett said.
Bassett will join her new teammates ahead of United's season-opening match on October 28 against Melbourne Victory.
The former Arsenal and Chelsea defender has heard plenty about the Australian league.
"I am eager to start training with the team so I can fully immerse myself in the playing philosophy and culture of the club," Bassett said.
"I have only heard impressive things from players who have played in the W-League; positives such as the competitive fixtures, different playing styles and the players' commitment and professionalism."
Bassett will form part of a new-look defensive line under new coach Heather Garriock featuring recently signed goalkeeper Haley Kopmeyer and star Matildas fullback Ellie Carpenter.
Garriock said Bassett would be an integral part of the backline.
"Laura will be the backbone to our defence this season," Garriock said.
"She can either play centre-back or holding midfield, and having a solid spine is key for this season."
Bassett was an unused substitute earlier this week as the Lionesses began their FIFA World Cup qualifying with a 6-0 thrashing of Russia.