The couple, who have only been dating for eight months, are determined to take their relationship to the next level and are also planning to adopt a child, reported Contactmusic.
"They want to get married very soon. Charlize always intended to have another child," a source said.
The 38-year-old beauty already has two-year-old son Jackson.
Penn, who also has 23-year-old daughter Dylan and 20-year-old son Hopper with his ex-wife Robin Wright, apparently has also been bonding with Theron's young son.