The dispute centered on the club's refusal to pay back refundable deposits to people who wanted to drop their memberships.
Plaintiffs said Trump owed them a total of $4.85 million. Judge Kenneth Marra added almost another million in interest, for a total of $5.7 million in his ruling issued Wednesday.
The country club is in the town of Jupiter, 90 miles (145 kilometers) north of Miami.
The case was heard in West Palm Beach, near Jupiter.
The city is home to Trump's vast Mar-a-Lago estate. The president is scheduled to go there this weekend, when demonstrators have warned they will stage protests against him.
Lawyers for the Trump Organization did not respond to queries from AFP about the breach of contract dispute.