The ruling, watched by many in the US, will require media conglomerate Grupo Clarin to divest some of its units. The government has championed the reform as the start of a new era of media diversity.
A statement from the court said it had deemed four clauses that it was reviewing in the anti-monopoly broadcast law to be within the terms of the constitution.
"A law that set limits a priori is legitimate because it favors freedom of speech by limiting market concentration," the court's ruling said.
Clarin, the country's biggest media group, has argued that the law's most controversial clause, Article 161, violates the constitution by forcing companies to sell off previously acquired radio, television or cable TV operating licenses. The law has been tied up in Argentina's courts for years.
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