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.