James H. Simons usually shuns publicity, though some attention is inescapable for a hedge fund billionaire and major philanthropist. But on occasion, he allows one cause to draw him into public view: Stony Brook University, where he was a mathematics professor long ago.

He joined two governors, David A. Paterson and Andrew M. Cuomo, in pressing the Legislature to shore up the finances of the State University of New York, the parent system of Stony Brook, in part by raising tuition. He made it clear that he could be very generous if the state acted, and this year lawmakers made the kinds of changes he sought.

On Wednesday, Mr. Simons, 73, and his wife, Marilyn, will announce the biggest gift by far in SUNY’s history, $150 million to Stony Brook. It is the sixth largest donation ever made to an American public university, according to The Chronicle of Higher Education, and is twice as large as the previous record for a gift to a public university in New York — the $60 million that the Simonses’ foundation gave to Stony Brook in 2008.

Most of the money will go to research in medical sciences, including the construction of a life sciences building and the creation of a neurosciences institute and a center for biological imaging, as well as to the study of cancer and infectious diseases.

Other portions of the gift will help pay for 35 new endowed professorships, and will create 40 fellowships for graduate students.

Dr. Samuel L. Stanley, president of Stony Brook, said that the donation, to be given over seven years, was more than triple the amount the university typically raised from all donors in a year. The university is 54 years old, he said, but “this is our rebirth day.”

Like most of the nation’s public universities, SUNY has faced declining state support, but unlike most of them, SUNY cannot compensate by raising tuition. In New York, the Legislature sets tuition at state universities, and for years it has resisted the rapid increases seen around the country.

That made SUNY a bargain, but it also squeezed the university’s budget, resulting in fewer class offerings, larger class sizes and inadequate facilities. Administrators worried aloud that the system would fall behind its peers.

“There was no point to continue to support an institution that the state was backing away from,” Mr. Simons said in an interview at the Simons Foundation’s offices near Madison Square in Manhattan. “Inexpensive access to mediocrity is not doing anyone such a favor.”

This year, the Legislature and Mr. Cuomo enacted a law that allowed SUNY and the City University of New York to raise tuition by $300 a year for five years and also committed the state to maintaining its support for the universities. Full-time, in-state tuition at SUNY is set to climb to $6,170 in 2014-15, up from $4,970 last year, and for the first time, the system’s four main research campuses — Stony Brook, Binghamton, Albany and Buffalo — can also charge somewhat more than its smaller universities to out-of-state students.

Dr. Stanley said the Simons gift was being tailored to the areas in which Stony Brook lags noticeably behind the most prestigious public universities around the country: the student-to-faculty ratio, the number of endowed professorships and the life sciences.

“We want to be a top-25 research university,” he said. “There’s no reason why New York shouldn’t have that kind of flagship public university.”

Public universities generally raise far less money from donors than private universities do, but Mr. Simons said he saw support for public schools as vital, “especially in these days when private universities are so unbelievably expensive.” As for Stony Brook, he said, “We have a warm spot in our hearts for that particular institution.”

Marilyn Simons, the president of their foundation, went to Stony Brook as an undergraduate and received a doctorate in economics there. Mr. Simons, who earned a bachelor’s degree in mathematics from M.I.T. and a Ph.D., also in mathematics, from the University of California, Berkeley, was appointed chairman of Stony Brook’s math department in 1968.

But in 1978, he left academia for finance, becoming one of the first highly successful “quants” — quantitative analysts who use sophisticated math to guide investments. In 1982, he founded Renaissance Technologies, a hedge fund firm; though he stepped down as chief executive two years ago, he remains chairman.

In October, Forbes magazine estimated his worth at $10.6 billion, making him the 30th wealthiest person in the country.

Now semi-retired, he spends more time on charitable work.

“It is fun,” he said, “but it’s not as much fun as it seems like it would be, since we have to say no a lot.”