The Oscar-winning couple are spending lot of time together and they seem to be inseparable, reported E! Online.
“They have made a commitment to be together for the rest of their lives. They are discussing when and where they will have a wedding,” sources said.
Theron, 39, and Penn, 54, spent their last Christmas together in Hawaii and both returned to Los Angeles on New Year’s Day.
Penn has been married twice. His first wedding was with singer Madonna, which lasted for four years. He then married actress Robin Wright with whom he has two children, daughter Dylan Frances, 23, and 21-year-old son Hopper Jack. Penn and Wright got divorced in July 2010.
Theron was earlier engaged to Irish actor Stuart Townsend for nine years, before they broke up in 2010.