Selig said in a statement that the TV contract would not be in the best interests of baseball and would further divert team assets to McCourt's "personal needs."
"Given the magnitude of the transaction, such a diversion of assets would have the effect of mortgaging the future of the franchise to the long-term detriment of the club and its fans," Selig wrote.
McCourt has struggled to meet team payroll since the start of the season. Without money from a TV deal, Selig could seize control of the Dodgers if McCourt doesn't pay his bills.
Up next is a June 30 obligation, where deferred compensation for some former players is due. Among them is Manny Ramirez, who is owed nearly $7 million as part of a two-year, $45 million contract.
McCourt attorney Steve Susman called Selig's decision "potentially destructive" to baseball and the Dodgers and said he plans to "explore vigorously our options and remedies."
"We are extremely disappointed with the commissioner's rejection of the proposed Fox transaction which would inject $235 million into the Los Angeles Dodgers," Susan said. "As Commissioner Selig well knows, this transaction would make the Dodgers financially secure for the long term and one of the best capitalized teams in Major League Baseball."
Matthew Hiltzik, a spokesman for McCourt's ex-wife, Jamie, declined comment.
The McCourts reached an agreement last week that was contingent on Selig's approval of a TV deal with Fox reported to be worth as much as $3 billion.
Under the settlement, Frank McCourt would receive $385 million up front. However, the settlement terms showed about $150 million would be used toward paying attorneys' fees, existing debt and an account that would be monitored by the divorce judge.
The Dodgers' current TV deal with Fox expires in 2013.
Selig said in his statement that he sent a letter to McCourt in which "expansive analysis" for his rejection of the TV proposal was explained. Some legal observers have said Selig's refusal to approve the TV deal would pave the way for a lawsuit by Frank McCourt against MLB.
On Friday, Frank McCourt said he had met criteria set forth by MLB in order for the TV contract to be approved.
The McCourts have been embroiled in a contentious divorce where their lavish spending habits were exposed. Court documents show they took out more than $100 million in loans from Dodger-related businesses.
In April, the league took the extraordinary step of assuming control of the troubled franchise. Former Texas Rangers President Tom Schieffer was appointed to monitor the team.