South Brother Island, seven acres of dense forest, bittersweet vines, flocks of wild birds and little else, is a speck in the East River — and a glimpse of what the rest of the city might have looked like thousands of years ago.

Historically overlooked and unwanted, it changed hands for $10 in 1975, despite being located in the middle of New York City.

But South Brother, the last East River island of any significant size to remain in private hands, will finally get its due today when it is formally transferred to the city as part of a complex $2 million deal brokered by the Trust for Public Land and financed with federal money secured by United States Representative José E. Serrano.

“The idea of buying an island — I mean, how many people get to buy an island?” said Mr. Serrano, whose district includes South Brother.