KR Media, launched last year by the former Aegis Media executives Bruno Kemoun and Eryck Rebbouh, had earlier won a ruling against Aegis, after the judge found that a raid of KR's premises was based on un-substantiated allegations of client poaching.
Aegis had been ordered to return any evidence obtained and to pay costs for each day that it held on to documents. However, the new ruling, at the Versailles court, overturns the former judgment and allows Aegis to keep any evidence.
The judge ruled last week that Aegis had "legitimately justified" that it had a possible case for breach of contract and unfair competition.
The raid on KR's Paris office was conducted in December by court-appointed officials. The dispute centres on whether Kemoun and Rebbouh broke a "non-solicit" agreement that they signed when they left Aegis.
The case is one of the first challenges to face Aegis' new chief executive, Robert Lerwill. A spokesman said Aegis was "very pleased". Kemoun was unavailable for comment.