Mark Hinueber, attorney for Stephens Media and the Review-Journal, declined immediate comment.
The Stephens Media buyout offer was accepted this month by a majority of the trustees of the Greenspun family trust that controls the Sun and its Internet properties.
Brian Greenspun voted no.
The deal, which would be effective Sept. 1, would turn over to Greenspun entities that already control the travel booking website Vegas.com ownership of the Stephens Media website lasvegas.com.
"The terms ... contemplate that the Las Vegas Sun will cease operations as both a print and online newspaper," Greenspun's lawsuit says.
The complaint invokes the federal Newspaper Preservation Act of 1970, the Sherman and Clayton antitrust acts and Nevada's state Unfair Practice Act. It seeks unspecified damages, a preliminary court order to block the Stephens Media deal with Greenspun trustees and a permanent injunction to prevent the termination of the joint operating agreement.
Greenspun attorney Leif Reid said Wednesday that the lawsuit seeks to block a monopoly, and has little to do with the willingness of other trustees to accept the Stephens Media offer.
Reid, a son of Democratic U.S. Sen. Harry Reid of Nevada, noted that the newspaper operating agreement provides the Review-Journal with protection against antitrust claims.
"The JOA was a 50-year commitment that both parties benefited from," he said. "They're not keeping the deal, and all gloves are off."
"When they seek to void the JOA, they no longer have protection under antitrust laws to do what they're trying to do," Reid said.
The newspapers have separate reporting staffs in separate locations, with the Review-Journal handling combined advertising and circulation. The Sun used to publish in the afternoon, but after the operating agreement was amended in 2005 they've been printed and distributed together in the morning.
The Sun has since developed a robust online presence, including its lasvegassun.com and Vegas.com.
The newspaper circulates some 252,000 copies on an average weekday according to March 31 figures from the Alliance for Audited Media. It won a Pulitzer Prize for public service in 2009.