They said this will amount to double the number of apprentices created each year - currently about 28,000.
All parties are promising to create more apprentices or equivalent schemes to get unemployment down.
Labour highlighted its pledge to create 100,000 additional apprentices.
The Lib Dems said they would fund their promise by moving cash from Jobs Growth Wales, the Labour Welsh Government's existing job-creation scheme.
Eluned Parrott, the Liberal Democrats' candidate for Cardiff Central, said: "Providing high-quality skills training is vital to boosting our economy and helping people to build careers and get on in life.
"Labour's own evaluation report said that Jobs Growth Wales locked people into low wages and that 73% would've found work without it.
"We all know this scheme is a failure, but Labour refuses to face the facts.
"The Welsh Liberal Democrats would instead invest in apprenticeships and training, which will actually give young people professional skills and help them get on in life."
On Labour's policy, Deputy Skills Minister Julie James said Wales already had "one of the best records in Europe" on apprenticeships.
"This pledge is the next step to ensuring that we have the skills base that projects like Wylfa Newydd and the South Wales Metro, will demand," she said.
"That is why so many voices within industry are so enthusiastic about this pledge."
Plaid Cymru has promised to created 50,000 new apprentices, while the Welsh Conservatives say they will lift the current 16-24 age restrictions on the Jobs Growth Wales scheme.
It helps firms employ 16-24-year-olds for six months by covering the cost of the minimum wage.
The Tories aim to replace it with a scheme called Journeys to Work, open to all ages, with jobs lasting for a year.
UKIP say they back industry-supported apprenticeships, along with more vocational options in post-14 education.
The Welsh Liberal Democrats would create 140,000 extra apprentices over the next five years if the party wins power in May's elections.