The decision by the Appellate Division in Manhattan means the Republican presidential nominee cannot comment publicly about individual prosecutors and others in the case until Justice Juan Merchan sentences him on September 18, seven weeks before the November 5 election.
Trump's lawyers did not immediately respond to a request for comment.
They have argued that the gag order violated Trump's constitutional free speech rights under the First Amendment.
A spokesperson for Manhattan District Attorney Alvin Bragg's office, which prosecuted Trump, declined to comment.
Merchan imposed the gag order a few weeks before the trial began on April 22, saying Trump's history of making threatening statements could undermine the proceedings.
The original order prevented Trump from commenting on prosecutors, court staff, witnesses and jurors.
A separate order against naming the anonymous jurors remains in effect.
Merchan lifted the restrictions on witnesses and jurors following Trump's May 30 conviction.
The Appellate Division, a mid-level appeals court, said threats that Bragg's staff received after the verdict continue to pose a "significant and immediate" threat.
"Justice Merchan did not act in excess of jurisdiction by maintaining the narrowly tailored protections," the five-judge panel said.
The appeals court upheld Merchan's original gag order in May, citing the need to protect people from "threats, intimidation, harassment, and harm," and rejecting Trump's First Amendment argument.
The order leaves Trump free to speak about Bragg and Merchan.
Jurors found Trump guilty on 34 felony counts of falsifying business records for having covered up former personal lawyer and fixer Michael Cohen's $US130,000 ($A198,700) payment to adult film actress Stormy Daniels.
The payment was made in exchange for Daniels' silence before the 2016 election about a sexual encounter she says she had with Trump a decade earlier, which Trump denied.
Trump won the presidency by defeating Democrat Hillary Clinton.
The criminal trial was the first of any US president.
Trump faces up to four years in prison, and can also be fined.
Time behind bars is rare for people convicted in New York for falsifying business records, especially those like Trump with no prior criminal histories.
Trump has vowed to appeal his conviction after he is sentenced.
