Atlantia, which runs more than 5,000 kms in toll motorways, said the total equity value for the whole of its Autostrade per l'Italia (ASPI) unit was 14.8 billion euros ($16 billion).
In a statement, the Italian infrastructure group said it had agreed to sell a 5 percent stake to a consortium 74 percent-led by Allianz Capital Partners and also including EDF Invest and DIF Infrastructure IV.
A further 5 percent was sold to the Silk Road Fund.
Atlantia said the Allianz-led consortium also had a call option to buy a further 2.5 percent of ASPI, on the same terms and conditions, before the end of October.
In a separate statement Atlantia confirmed its preliminary interest in a tie up with Spanish peer Abertis (ABE.MC), providing the deal was done on a friendly basis.
Reporting by Stephen Jewkes