The decision represented "the recognition of our children's right first and foremost to have the two mothers or two fathers who raised them acknowledged by the law," said Marinella Grassadonia, president of Famiglie arcobaleno (Rainbow families), which campaigns for gay adoption in Italy.
The two mothers in the case were married abroad and have lived in Rome since 2003. One conceived the child by artificial insemination, also outside Italy, and in August 2014 a youth court had granted the second woman the right to adopt their daughter.
This decision was overturned but successfully appealed by the couple, with judges invoking the "superior and pre-eminent interest of the child to maintain cohabitation and an affectionate relationship," according to the couple's lawyer, cited by local media.
Since the initial 2014 decision, Italian courts have authorized similar adoptions or recognized those that took place in other countries.
In January, a Turin court ruled that a woman married and then divorced in Spain should be considered equally the mother of her ex-partner's biological child.
While Italy is the last major western European country not to offer any legal recognition for same-sex couples, the government is working on a law that would legislate for civil unions for gay couples which would explicitly allow for a partner to adopt a child.
In July, the European Court of Human Rights upheld discrimination complaints filed by three Italian couples, saying Rome should introduce some form of civil union - an embarrassing ruling which helped pro-civil unionists.
Prime Minister Matteo Renzi has promised a conscience vote on the bill to lawmakers, which is expected to be considered in parliament by the end of January.
While attempts by previous administrations to change the law have been thwarted by parties with deep ties to the powerful Catholic Church, this time the majority of opposition parties have said they support the bill.
Grassadonia of Famiglie arcobaleno said Wednesday's legal decision was "only a first step" offering a minimum of protection, calling for the institution of full gay marriage and parental rights for same-sex couples from the birth of their children onwards.
A number of EU countries including Britain, France and Spain have legalized gay marriage, but other southern and eastern European states have made slower progress.
Greece's parliament on Wednesday approved a bill granting same-sex couples the right to a civil union, overcoming years of opposition from the influential Orthodox Church.
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