The Largest Public Library In The United States
"Largest public library" sounds like an easy trivia question. It is not. The words "public library" are doing a heroic amount of heavy lifting here, and depending on exactly how you define them, the answer changes completely. Worse, the answer most people blurt out first is, frankly, cheating. So let's sort out who actually deserves the crown, and hand out a few honorable mentions along the way.
The Library of Congress: Largest, but Not Public

Yes, the Library of Congress is enormous. It is the biggest library in the United States and, in fact, the biggest in the entire world, holding well over 167 million items, including nearly 40 million books plus maps, manuscripts, films, and the personal papers of presidents. Some sources will even flatly call it the country's largest "public library." But come on. The Library of Congress is the nation's de facto national library, built to serve the US Congress. You can visit it, and anyone 16 or older can get a reader card, but you cannot stroll in and check out a paperback thriller to take to the beach. It is the overachiever who technically qualifies for the contest and then refuses to play by the rules. For a real public library, we need somewhere that will actually lend you a book.
The New York Public Library

If you mean an actual public library, the kind with a library card, a due date, and a soul-testing hold list, the champion is the New York Public Library. With roughly 53 million items spread across 92 locations, it is the largest true public library system in the country. It is the one guarded by those two famous marble lions, Patience and Fortitude, which is conveniently the exact mood you will need while waiting for a popular new release to come off hold. Its Rose Main Reading Room is a jaw-dropping, block-long temple to reading, and unlike its cheating cousin down in Washington, the NYPL will happily let you walk out with a book, as long as you promise to bring it back.
Boston and Chicago: The Other Contenders

Two more contenders deserve a shout. First, the original: the Boston Public Library was the first large, free, tax-supported municipal library in the country, opening to the public in 1854 with the noble words "Free to All" carved right over the entrance. It still holds tens of millions of items and remains one of the most important libraries in America. And then there is the size-by-square-footage angle, which hands the trophy to Chicago. When the Harold Washington Library Center opened in 1991, it earned a Guinness World Record as the largest public library building on Earth, sprawling across more than 750,000 square feet and topped with enormous aluminum owls glaring down from the roof.
So who actually wins? It depends entirely on your definition, which is the single most librarian answer imaginable. If you want the biggest collection of anything and everything, it is the Library of Congress. If you want a genuine, book-lending public library, it is the New York Public Library. If you want the biggest single building, head to Chicago. And if you want to thank the place that started the whole radical idea of free books for everyone, tip your hat to Boston. Pick your lane, and there is a library standing by to win it.