Witherspoon, 35, married US talent agent Jim Toth on Saturday, and invited a host of celebrities to the ceremony – including the Iron Man 2 actress and her 50-year-old new beau.
A friend of Johansson's told PopEater.com: 'Scarlett is Jim's client, but no one, including the bride, knew she would have turned up with Sean Penn.
'Turning up at a wedding with your boyfriend is definitely a huge statement, especially that wedding. What it's doing is announcing to the whole of Hollywood that they are serious.'
Witherspoon and Toth married in a private ceremony at her ranch in California.
Penn and Johansson, 26, were first spotted together earlier this year at a bash in Los Angeles celebrating Black Swan.