Mediapro was fined 6.5 million euros, Real 3.9 million, Barca 3.6 million and Sevilla and Santander 900,000 and 30,000 euros respectively, the commission (CNMC) said on Monday.
The fines were levied because the broadcaster and the four clubs had agreed four-year contracts for TV rights, a year longer than the three years allowed.
Mediapro said it and the clubs had acted "in good faith" and the contracts complied with Spanish law.
The broadcaster will appeal the ruling while taking steps to ensure it does not have to pay the fines while a decision is pending.
Mediapro and Canal Plus, part of the Prisa media group, share the rights to broadcast La Liga matches in Spain.
Unlike in other major European leagues, the sale of TV rights is not subject to collective bargaining and clubs agree deals individually with broadcasters.
Real and Barca, the world's two richest clubs by income, between them take about half the total TV revenue for La Liga of around 700 million euros.
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