The new policy, announced in January, took effect Thursday.
Entrance will remain free for children under 12 and pay-what-you-wish for students in New York, New Jersey and Connecticut.
Students living outside the tri-state area will be charged $12 and seniors $17.
People from all over the world had been able to come to the museum for nearly nothing since its founding in 1870. But the number of people willing to pay a suggested donation of $25 dropped off substantially in recent years.
The New York Times says that initially, New York residents who show up without proper identification will be asked to bring it next time.