"Yes, we are excited to confirm that they are engaged!" said the rep. The couple were introduced by mutual friends and have been dating since May.
Simpson told People that the pair plan to spend Thanksgiving together with their families at the Macy's Thanksgiving Day Parade in New York.
"We're all gonna be in New York," said the pop singer. "I'm gonna be on a [Macy's parade] float. It's not always the best way to spend a Thanksgiving, but it's a great way to celebrate. So, maybe I'll just have all families on the float."
Simpson said her relationship with Johnson was "just normal" and said she "couldn't ask for a better man in my life right now."
Johnson has played for the New Orleans Saints and the San Francisco 49ers. In college, he was a wide receiver for Yale University.
Simpson's ex-husband, Lachey, whom she starred with in MTV's The Newlyweds, announced his engagement to Vanessa Minillo last week.
Catch the newly engaged couple, Simpson and Johnson, on the Macy's Thanksgiving Day Parade airing live Nov. 25 from 9 a.m. to 12 p.m. on NBC (KARK Ch. 4 in Little Rock).