The European Council confirmed that the official instrument of ratification was delivered on 10 February.
Both chambers of the Italian Parliament approved the ratification bill in September and October last year.
The UPC Agreement requires 13 countries to ratify before it takes effect. These must include the UK, Germany and France.
The UK has signed the UPC Protocol on Privileges and Immunities ahead of its ratification of the agreement.
Poland and Spain are the only two EU members not involved in the unitary patent system.
Spain has always stood firmly against the unitary patent due to language concerns.
Poland, on the other hand, was an original signatory. But the country announced in 2013 that it would not sign the UPC Agreement and would deliberately stay outside of the system.