The 26-year-old actress has been romantically linked to the 50-year-old actor in recent weeks and despite the rumours initially being denied, she was accompanied by Sean as she watched Reese tie the knot with Hollywood agent Jim Toth, who also looks after Scarlett.
A source told website PopEater: "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."
Scarlett – who announced her separation from husband Ryan Reynolds last year after two years of marriage – is allegedly completely smitten with Sean and wants everyone to know they are dating.
A source said: "Scarlett is head over heels in love with Sean. She normally tries to hide away from public attention, but when she's with Sean she doesn't care."
Scarlett and Sean first became friends when she was planning to fly out to Haiti to help victims of the two earthquakes which hit the Caribbean nation in January 2010.
A source said: "Scarlett first reached out to Sean when she was planning to visit Haiti with Oxfam. She knew Sean was living there in a tent and turned to him for advice."
Sean was previously married to Madonna, while his divorce from actress Robin Wright - mother of his two children Dylan, 19, and Hopper, 17 - was finalised in July 2010 after 14 years of marriage.