Party leader Leanne Wood said inequality had grown after decades of gradual erosion of pay and conditions.
She accused the Tories of "hostility" to trade unions and Labour of being "embarrassed" about its links to them.
Plaid Cymru has called for a "living wage" and more say for employees, and an end to zero-hours contracts.
The party has claimed increasing the statutory minimum wage of Â£6.50 an hour to the higher Â£7.85 living wage would boost pay for 250,000 people in Wales by 2020.
It added that the measure could create up to 20,000 jobs through higher spending in local communities.
Calling for a Royal Commission to examine wider workplace issues, Ms Wood said: "It would be an opportunity for communities, trade unions, businesses and the public to get involved in a conversation about the kind of conditions we all want to see for working people."
Low pay and workers' rights should be investigated by a Royal Commission looking at industrial relations, Plaid Cymru has said.