MERIDEN — Kimball Union Academy has received an anonymous gift of $20 million for its endowment.

It is the largest single gift in the private preparatory school’s 206-year history and equals more than half of the school’s existing $30 million endowment.

“It’s pretty emotional to get a gift of this size and think of how transformative it can be for a small school,” said Rob Snow, who chairs KUA’s current 19-member board of trustees.

Snow, who grew up in Bedford, said the gift is really key to the long-term sustainability of the school.

“It is my firm belief that (head of school) Mike Schafer’s leadership, a dedicated faculty, and outstanding students have inspired this generosity and we are humbled by it,” said Snow, who now resides in Bethesda, Maryland.

“With great anticipation I look forward to witnessing the impact that this gift has on our community and the future leaders who learn and grow at KUA,” said Snow, Class of 1985.

News of the gift spread quickly in the Upper Connecticut Valley community and among former KUA students.

“This is wonderful news for a wonderful school and will materially help in insuring Kimball Union to continue its important work,” said Warren D. Huse of Laconia, Class of 1955.

School spokesman Sarah Moore said the gift is a testament to the impact KUA has had on the lives of its students and community and represents an appreciation for the school’s mission to prepare students for the challenges of tomorrow’s world by inspiring academic mastery, creativity, responsibility and leadership.

She said the gift will have a “profound influence” on current and future generations of Kimball Union students and employees in a prepared media release.

Snow said the funds are unrestricted and will be placed in the endowment fund where they can be drawn on over time, but will allow for some immediate improvements to the physical plant — specifically, campus housing upgrades, to help attract and retain talented and motivate employees, and for scholarships.

“Diversity is important to the character of the school,” Snow said, noting the gift will increase accessibility for students.

Established in 1813, KUA is one of the nation’s oldest boarding schools.