AM Simon Thomas said it would use Wales' share of proceeds from UK ministers' apprenticeship levy, around £150m a year, to fund the plan.
He said it would boost young people's prospects and plug "skill gaps".
On Tuesday, Welsh Labour ministers outlined plans to spend an additional £5m on apprenticeships in 2016-2017.
Mr Thomas launched the proposals with Plaid leader Leanne Wood at a gas training academy in Tredegar, Blaenau Gwent.
Although aimed at young people, Plaid Cymru said it would put no age restrictions on its proposed scheme.

Three converted first-half tries from Cory Hill, Ashton Hewitt and Jason Tovey gave the Welsh side a 24-0 lead at the break that did not flatter them.
Carl Meyer crossed after the break for a fourth try to secure the bonus point.
Paddy Butler, Mosese Ratuvou and Watisoni Votu all added tries for Pau but the Dragons were always in control.
Victory leaves the Dragons top of Pool Two, two points above second-placed Sale Sharks after their win over Castres.
Second row Hill's brilliant individual try gave the visitors an early lead, before a Tovey penalty made it 10-0 and then Sarel Pretorious' pass saw Hewitt burst through a gap in the Pau defence to cross for the second try.
Tovey ran clear after a mistake by the French side to add the Dragons' third try and he duly converted, before kicking through to Meyer for the fourth try after the break.
With Hill sent to the sin-bin, Pau capitalised to score through Butler, but they failed to convert and a Tovey drop-goal made it 34-5.
Word Cup-winning New Zealand centre Conrad Smith came on for his Pau debut and, shortly afterwards, Ratuvou barged his way over the line for the home side's second try.
Fijian Votu added a late third score but Pau remain bottom of the pool.
Dragons director of rugby Lyn Jones told BBC Radio Wales:
"We're absolutely delighted. It was a great performance from us. It was as good as we get
"We were a far better team than them and we deserved to come away with the spoils.
"We kept (Colin) Slade quiet and they couldn't deal with us."
Pau: Damien Traille; Sireli Bobo, Julien Fumat (c), Jale Vatubua, Mosese Ratuvou; Colin Slade, Thierry Lacrampe; Jérémy Hurou, Mehdi Boundjema, Sylvain Charlet; Daniel Ramsay, Julien Pierre; Josefa Ulago Domolailai, Paddy Butler, Brice Monzeglio.
Replacements: Quentin Lespiaucq Brettes, Julien Jacquot, Euan Murray, James Coughlan, Sean Dougall, Thibault Daubagna, Conrad Smith, Watisoni Votu.
Newport Gwent Dragons: Carl Meyer; Nick Scott, Adam Hughes, Adam Warren, Ashton Hewitt; Jason Tovey, Sarel Pretorious; Phil Price, Elliot Dee, Lloyd Fairbrother; Cory Lewis Hill, Rynard Landman (c); Nick Crosswell, Nic Cudd, Taulupe Faletau.
Replacement: Rhys Buckley, Brok Harris, Shaun Knight, Matthew Screech, Ollie Griffiths, Charlie Davies, Dorian Jones, Ross Wardle.
Referee: Andrew Brace (Ireland)
Touch judges: Frank Murphy (Ireland), Stuart Douglas (Ireland)
TMO: Kevin Beggs (Ireland)
Citing commissioner: Dave Guyan (England)
Plaid Cymru would create 50,000 apprenticeships if it wins power in May's assembly election, the party has announced.
[NEXT_CONCEPT]
Newport Gwent Dragons enjoyed a comfortable bonus-point win in Pau to edge one step closer to reaching the European Challenge Cup quarter-finals.