The Czech fashion model called off her wedding plans with the British actor last month. The couple had been together for about two years.
"Petra started going on dates with Sean right after the split," an insider told UsMagazine.com.
Nemcova, 32, and Penn, 51, first dated for a few weeks in 2008.
Nemcova was previously engaged to Simon Atlee, who died in the 2004 tsunami that hit South Asia where the pair were on holiday.
Penn has two adult children with ex-wife actress Robin Wright. They were a couple for more than 20 years before their divorce was finalized in July 2010.