The show has received a 10-episode order at the studio for premiere in 2019, the NBC said in a statement.
The series will be produced by Universal Television and executive produced by Fellowes and fellow Downton executive producer Gareth Neame.
It is set in 1880s New York City, a period of immense social upheaval, of huge fortunes made and lost, and of palaces that spanned the length of Fifth Avenue.
The series will focus on Marian Brook, a wide-eyed young scion of a conservative family who will embark on infiltrating the wealthy neighbouring family dominated by ruthless railroad tycoon George Russell, his rakish and available son Larry, and his ambitious wife Bertha, whose "new money" is a barrier to acceptance by the Astor and Vanderbilt set.
"I have been fascinated by this period of American history for many years and now NBC has given me the chance to bring it to a modern audience. I could not be more excited and thrilled," he added.
"The Gilded Age" marks Fellowes' follow-up to "Downton Abbey", which ran for six seasons and is the most-nominated non-US show in Emmy awards history, with 69 nominations and 15 wins.