Barcelona have been fined 500,000 euros for wrongly reporting "profits on disposal of intangible assets" in 2022.
United have been fined 300,000 euros for "minor break-even deficits".
"While disappointed by the outcome, Manchester United accepts this fine for what UEFA acknowledges to be a minor technical breach of its previous Financial Fair Play rules," United said in a statement to British media.
Cypriot side APOEL and Turkey's Konyaspor were fined 100,000 euros each for the same offence as United. Turkish clubs Istanbul Basaksehir and Trabzonspor, Belgian champions Royal Antwerp and Anderlecht were among other teams to receive fines.
The decisions follow the last round of monitoring using FFP rules that have been replaced by new sustainability regulations announced last year.
A group of high-profile European clubs including Paris St Germain, AC Milan and Inter Milan, who agreed settlements with UEFA in September last year, have fulfilled targets set for the financial year 2022.
UEFA added that these clubs will continue to be monitored for compliance next season.
