Plaid Cymru has been fined £29,000 for failing to accurately report cash donations.
An Electoral Commission investigation found the party submitted nine inaccurate reports over a two year period, omitting a total of 36 cash donations worth more than £497,000.
The party reported the donations in May 2018.
Plaid Cymru said the matters highlighted were an "oversight" and once highlighted were "immediately corrected".
The Electoral Commission found "a lack of effective internal processes" within the party had led to a "failure to understand" the reporting requirements.
A spokesperson for the regulator said the reporting failures "led to a disappointing lack of transparency".
Louise Edwards, Director of Regulation, said:“The total number and value of donations omitted from Plaid Cymru’s quarterly reports is significant and reveals a substantial degree of non-compliance. Plaid Cymru is a well-established party and it should be able to meet its reporting obligations.
“It is vital that voters can see full and accurate financial records that show where a political party’s money comes from. Plaid Cymru’s sustained failure over a two-year period to understand and meet its legal duties led to a disappointing lack of transparency into the party’s finances.
“Voters can have confidence that, where parties fail to comply with the rules and there is no reasonable explanation, we will take action.”
A Plaid Cymru spokesperson said: “The matters highlighted by the Commission are historic in nature and do not involve any private donations.
"As soon as Plaid Cymru were notified, the Party immediately corrected this oversight and is now in full compliance.
"All public funds have been fully reported in the Party’s Annual Report.”