The plan had been revealed in the party's manifesto, but city Labour leader Gordon Matheson gave further details of the promise when he visited the Four Seasons nursery in Waterloo Street.
He told parents that if Labour was re-elected as the city's ruling party it would extend free childcare by up to five months, with the scheme being introduced in August.
Labour's manifesto also said it would extend free childcare under the Vulnerable Twos programmes, which supports toddlers across the city.
The party has also promised to open a further 10 family learning centres across the city and to extend training for early years staff aimed at helping parents into work.
There are also plans to extend nurture classes – usually about eight to 10 children – in the early years of primary.
Mr Matheson said: "Glasgow was the first council in Scotland to offer free childcare to three-year-olds, but our city must stay ahead of the game.
"Childcare is vital in helping parents, especially women, get back into the jobs market.
"The SNP says it can't offer extra childcare until after the referendum, but Labour is doing it right here, right now.
"Our plans will be a massive boost for children, parents and our city. One of the first things the SNP did after forming a government five years ago was to scrap an important scheme aimed at expanding childcare to vulnerable two-year-olds. That was a disgraceful decision and Labour will bring it back."
The SNP says if it is elected as the city's ruling party it will work with the Scottish Government to give children at least 600 hours of free nursery education per year.