The sale is subject to the approval of the Department for Transport (DfT).
The DfT is expected to announce a decision in February.
National Express is selling the franchise for around £70mln, resulting in a small net profit for the UK company.
It could mean the end for National Express’s train business in the UK.
While not ruling out participating in future UK rail contracts, National Express has seen significant growth in North America and continental Europe in recent years and it believes there are further opportunities for growth in these markets.
“The board believes the transfer of c2c to Trenitalia UK presents opportunities for all concerned,” said Dean Finch, chief executive officer of National Express.
"Trenitalia has significant rail credentials and is excited by the opportunity to invest in the new agenda set out by the Secretary of State,” Finch added.
Shares in National Express eased a couple of pence to 346.5p in the first hour of trading.