Lawmakers in the lower-house Chamber of Deputies voted 372-51 in favour of the legislation with 99 abstentions after an earlier vote of confidence in the government on the issue had been equally comfortably carried, making approval of the bill itself automatic.
The long-awaited and much-disputed legislation was hailed as a landmark but also criticised as falling short of full equality for gay couples, particularly in relation to adoption and marriage rights.
"We hope the next law, which we will all fight for ... will be for marriage and adoption. And I hope there will be also a law against homophobia," she told AFP.
Monica Cirinna, the senator who was the main author of the bill, said she expected the first civil union ceremonies "no later than September" and dismissed opponents who vowed to seek a referendum aimed at overturning the law.
"We will welcome that with open arms. It will allow us to push on to equal marriage even sooner," she said. "Italy will reject medieval bigotry and conservativism."
"What mother or father would attend a party their children are not invited to?" she said.
Prime Minister Matteo Renzi, who had backed the bill but largely stayed out of the debate, called the confidence vote to short-circuit potential last-minute blocking or delaying amendments by opponents of the legislation, who include rebels in his own party as well as the Catholic right.
"Today is a day of celebration for so many people," the centre-left leader wrote on his Facebook page, framing the vote as another victory for his reform programme.