The RAC Foundation survey, details of which were revealed earlier this week, found 361 out of 6,694 council-maintained bridges across the country did not meet Department for Transport standards for capacity and weight restrictions, which means they are not fit to carry the heaviest vehicles on the roads.
Newport had the highest proportion of substandard bridges (30%) followed by Denbighshire (22%) and Conwy (20%).
Philip Gomm, the RAC's head of external communications, said bridges were not "about to collapse" but the survey highlighted how many were incapable of supporting large vehicles such as 44-tonne lorries.
"These structures, many of which will have been built hundreds of years ago, are under relentless and growing pressure from both human activity and the elements," he said.
The RAC estimated the cost of bringing all of Wales' bridges up to a good standard would cost about £98m.
A spokesman for the county council said: “The figure for Denbighshire includes all bridges that have a sub-standard structural / scour assessment or where inspections have identified significant (capital sized) works. All these bridges are being monitored, and restrictions are only being applied where necessary.
“The information has enabled us to identify works necessary to make sure that our bridges remain operational and we have a 10 year investment plan to address those works.
“We also made a substantial additional investment in the revenue budget for highway structures last year, which is aimed at ensuring they are adequately maintained and remain operational for the foreseeable future.”