The deal is for one year and worth up to $2 million, a source told ESPN's Adam Schefter.
Hall, 31, visited the Giants for a second time on Wednesday. They wanted another look after his offseason back surgery. Hall impressed, and was choosing between the Giants and the Bengals. He had spent the first nine years of his career with the Cincinnati Bengals.
Hall is expected to compete immediately as the Giants' slot cornerback. Trevin Wade has been running with their first-team defense this summer at that position.
The Bengals' first-round pick in 2007, Hall has started 121 games. He's been used primarily as a slot cornerback the past few seasons.
Injuries marred much of the latter years of Hall's time in Cincinnati. He tore both of his Achilles in separate incidents during the 2011 and 2013 seasons. Although he returned remarkably strong following the first tear, he has been a little slower getting back to form since the second one.
Neither Hall nor his Bengals coaches would say if he had lost a step after the second injury, but his role was scaled back. Hall played strictly at the slot corner position in 2015 after splitting more time there and on the boundary at earlier times in his career.
In a corresponding move, cornerback Tramain Jacobs was waived by the Giants.
ESPN Bengals reporter Coley Harvey contributed to this report