The 35-year-old American will play her final professional match on Sunday, when her San Diego Wave side face the North Carolina Courage in the National Women's Soccer League.
Morgan, considered among the best USA women's national team players of all time, finished her international career with 123 goals.
She took to Instagram to share her decision in a tearful update, saying: "I have so much clarity about this decision and I'm so happy to be able to finally tell you.
"It has been a long time coming and this decision wasn't easy, but at the beginning of 2024 I felt in my heart and soul that this was the last season that I would play soccer."
The striker was part of the USA women's teams that won the World Cup in 2015 and 2019 and Olympic gold at London 2012, as well as a key figure in the Americans' off-pitch advocacy efforts and legal fight for equal pay.
She was left out of Emma Hayes' squad for this summer's Paris Olympics, where the United States won gold.
Morgan gave birth to her first child, daughter Charlie, on May 7, 2020, and with her goal against Brazil on February 22, 2023, she moved past Joy Fawcett to become the all-time leader in USWNT goals as a mother with 14.
She added: "This is also not the retirement video I expected when I initially thought I was going to do this, because Charlie is going to be a big sister.
"I'm pregnant, and as unexpected as this came we are so overjoyed."
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