The outdoor performance center announced the gift late Friday, about 48 hours before its new, $9 million complex opens Sunday evening.
In a news release, the Levitt said the money would anchor its ongoing fundraising campaign. It includes a $2.5 million capital gift, plus a structured loan that converts to a $2 million gift over a period of 20 years.
The new venue on the banks of the Saugatuck River opens Sunday evening with a concert by guitarist and singer-songwriter Jose Feliciano.
The Levitt said it would formally announce a public fundraising drive at the Feliciano concert. The public phase of the drive will be called "Because Summer Lives Here," according to the release.
The Feliciano concert is free, as is the majority of the pavilion's performances.
The performance center is named for the late Mortimer Levitt, who died in 2005 at age 98, and his wife Mimi, longtime Westport summer residents and primary donors. Mimi Levitt continues to serve on the pavilion's board and still attends selected events.
"This is a momentous occasion in every way," Freda Welsh, the Levitt's executive director said in a news release. "Mimi's dedication to the original Levitt Pavilion and the Town of Westport has resulted in an extraordinary home for the performing arts that will instantly be recognized as an iconic cultural destination. We are so very grateful."