It will call for a fund to be created to allow local councils and community groups to make free parking available, in an assembly debate on Wednesday.
Plaid shadow local government secretary Sian Gwenllian said the move would help "level the playing field" with out of town developments.
Ministers called the link between car parking and visitor numbers "complex".
Ms Gwenllian said figures for the number of people visiting town centres were worrying "with town centres such as Abergavenny, Mold and Aberystwyth experiencing a decline in footfall of 39%, 28% and 18% respectively".
"We know that the lack of free parking is often a big turn-off for shoppers," she said.
"Out-of-town developments often offer free parking so it makes it very difficult for the High Street to compete.
"By allowing local authorities and community groups to provide free parking for shoppers, we can help level the playing field for high streets and get shoppers back there."
Ms Gwenllian said Plaid wanted the Welsh Government to set up a fund to help local authorities cover the cost.
A Welsh Government spokeswoman said: "We recognise the importance of high streets and are investing around Â£145m in supporting them through our Vibrant and Viable Places programme, Business Improvement Districts, town centre partnerships and town centre loans scheme.
"Car parking charges are a matter for local authorities, however, last year, we commissioned research into the effects of free parking on town centre footfall, which highlighted that the relationship between car parking and footfall is complex, as did the National Assembly for Wales Enterprise and Business Committee review."
She added that local councils could use the research to inform their decisions.
Meanwhile, Wrexham council has announced it will provide free parking at all council-run car parks in December.
Free parking should be offered in town centres across Wales to boost the High Street, Plaid Cymru has said.