Saracens were fined 50,000 euros (42,458.7 pounds) on Friday, but escaped expulsion from the Champions Cup, for fielding an ineligible player during a match last month.
European Professional Club Rugby (EPCR) had brought a misconduct complaint against Saracens after the club informed the governing body that prop Titi Lamositele may have been ineligible when he played against Racing 92 in round six.
Lamositele, 24, was eligible to play when the Premiership side announced their squad for the match on Jan. 19 but Saracens said the American's work permit had expired the day before the game.
Lamositele came on as a replacement in the 60th minute of the final pool game as defending champions Saracens won 27-24 to advance to the quarter-finals.
An independent disciplinary committee imposed the fine at a meeting in London, with half the sum suspended until the end of the 2020/21 season.
"The Committee believes that this was an unfortunate sequence of events brought about by an administrative oversight," said committee chair Roger Morris.
"However, Saracens are in breach of the disciplinary rules of the Heineken Champions Cup, and that breach constitutes misconduct which can be proportionately dealt with by way of a financial penalty."
The Premiership champions were relegated from English rugby's top division the day before the Champions Cup match for breaching salary cap rules.
The club's interim chief executive Edward Griffiths also resigned late last month after the club accepted their imposed relegation at the end of the season.
(Reporting by Rohith Nair in Bengaluru/Alan Baldwin; Editing by Christian Radnedge/Pritha Sarkar)
