In a statement, it said that both will now now move to finalise the transactional documents as soon as possible.
The conclusion of the investment is subject to final regulatory approvals, it said.
Alitalia voted on June 13 to accept Etihad's offer to invest in the company. The airline is sitting on debts of about 800m euros.
According to Reuters, Italy's flagship airline has made an annual profit only a few times in its 68-year history and received numerous state handouts before being privatised in 2008. It was kept afloat by a government-engineered 500 million euro ($680 million) rescue package last year but risks having to ground its planes unless a deal can be struck with cash-rich Etihad to allow it to revamp its flight network.