The spokesperson said the event on Martha's Vineyard will now include only family and close friends.
Hundreds of former administration officials, celebrities and Democratic donors were originally slated attend the party.
Obama's office declined to give a new estimate of how many guests will attend the gathering.
"This outdoor event was planned months ago in accordance with all public health guidelines," spokesperson Hannah Hankins said in a statement.
That included requiring guests to submit proof of a negative COVID-19 test and an on-site COVID coordinator.
President Joe Biden is not planning to attend the gathering, according to a White House official.
"While President Biden is unable to attend this weekend, he looks forward to catching up with former President Obama soon and properly welcoming him into the over 60 club," the administration official said. Biden is scheduled to spend the weekend at his home in Rehoboth Beach, Delaware.
The Obamas have identified various charities for guests to consider supporting, rather than giving birthday gifts, including My Brother's Keeper Alliance, the Girls Opportunity Alliance, and the Obama Foundation's Global Leadership programs.
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ABC News contributed to this report.