The veteran American will arrive in November on a three-month deal, meaning she will be available for the opening nine matches of the season.
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McDonald won the World Cup with the United States in 2019, and has scored four goals from 19 games for her country.
The 34-year-old played with Melbourne Victory during the 2012-2013 ALW campaign, and she’s looking forward to her return to Australia.
“I am super stoked to have another opportunity to play in Australia,” McDonald said in a statement.
“It’s always a great opportunity as an athlete to test your soccer knowledge and continue to learn in different ways, adjusting to the game how it’s played elsewhere.
“Hopefully I can help another program succeed with what I can bring to the table.”
Western United’s addition to the ALW means the competition will expand to 11 teams next season, with the Central Coast Mariners joining in 2023-24.