Macario is one of the world's most exciting young players and has just completed a move to Chelsea after a highly successful spell at Lyon.
The 23-year-old joined the Women's Super League winners last month on a free transfer after two seasons in France.
"Cat is one of the most intelligent, creative attacking players in the world," said Blues manager Emma Hayes.
"She brings a great deal of flair, European experience, and an unbelievable winning mentality. I think our fans will really love her."
Macario starred for Lyon in the 2021/22 season and became the first American to score in a Women's Champions League final as they won a record eighth title with a 3-1 win over Barcelona.
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But the attacking midfielder has been not selected in the USWNT's squad for their World Cup defence in New Zealand and Australia this summer.
Why is Macario not playing?
Macario has not played for over a year after rupturing her anterior cruciate ligament in the final game of 2021/22 Division 1 Feminine.
The game was Lyon's final match of the season but Macario was subbed just 13 minutes into the 4-0 win over Issy in June 2022.
There has been no official date on her comeback, but the Brazilian-born USA star ruled herself out of the World Cup in May.
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In a statement posted to social media, she wrote: "While I'm excited and superb optimistic about my future as a footballer, I'm sad to share that I won't be physically ready for selection to our U.S. World Cup team.
"The desire to return to play for my club and country has driven my training and fueled my everyday life. However, what's most important right now is my health and getting fit and ready for my next club season.
"I am eternally grateful to Aspetar Orthopaedic and Sports Medicine Hospital for always supporting me and guiding me in my rehab, and to U.S. Soccer and Vlatko Andonovski for prioritizing my long-term health over any individual and collective ambitions.
"I'll be cheering hard for my team-mates at the World Cup and I look forward to dedicating myself to fight to earn a spot for what I hope will be a long future on the U.S. national team."