Plaid Cymru has been fined £29,000 by the Electoral Commission for finance reporting failures.
The watchdog imposed the levy after the Welsh party submitted "nine inaccurate reports" regarding donations.
A spokesman for the Electoral Commission said: "The investigation found that 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.
"The commission's investigations pointed to a lack of effective internal processes, which led to a failure to understand the reporting requirements."
The commission's director of regulation Louise Edwards 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."
The commission said Plaid Cymru must pay a fine of £29,268 by March 2.
