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.
Plaid Cymru would create 50,000 apprenticeships if it wins power in May's assembly election, the party has announced.