The entertainment website said Simpson, 29, and Johnson, 30, began dating back in May. A source close to TMZ described the new couple as "very happy."
The Dukes of Hazzard star famously dated Cowboys quarterback Tony Romo and singer John Mayer after her split with husband Nick Lachey in 2006.
Johnson was drafted by the 49ers in the seventh round of the 2001 NFL Draft after attending Yale
Simpson shot to fame with Lachey when they appeared together in the MTV reality show "Newlyweds: Nick and Jessica."