The singer was seen showing off her engagement ring at Dillard's department store in Kansas City, Mo., Saturday as she was promoting her clothing line, People magazine reported.
The couple began dating in May, after being introduced by friends.
Simpson, 30, recently told People magazine she and Johnson, 31, were planning to spend Thanksgiving together.
"We're all gonna be in New York," said the entertainer and fashion designer. "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."
Johnson has played for the San Francisco 49ers and the New Orleans Saints. He broke 11 school records while playing wide receiver for Yale University and is also a former academic All-American.
A few days ago, Simpson congratulated ex-husband Nick Lachey on his engagement to Vanessa Minnillo, saying, "I couldn't be more happy for Nick."
Simpson and Lachey split in 2005 after three years of marriage.