The county filed the appeal with the 9th U.S. Circuit Court of Appeals after the Board of Supervisors voted 4-1 to authorize the legal action, the Imperial Valley Press reported online Tuesday.
Voters in Imperial County supported Proposition 8 by a large majority in the November 2008 election, the Los Angeles Times said.
U.S. District Chief Judge Vaughn R. Walker -- who ruled last week the same-sex marriage ban violated the U.S. Constitution -- has previously rejected efforts by Imperial County to intervene in the case, the Times reported.
The county is being represented by Advocates for Faith and Freedom, a Christian law firm based in Murrieta, Calif., that says on its Web site it is "dedicated to protecting religious liberty in the courts."
In a statement posted on its Web site, the group said it would "fight hard on behalf of California voters to ensure that their votes on this initiative, and future initiatives, are respected by the judiciary."
"California voters have the right to vote pursuant to their religious and moral beliefs," the statement said. "Judge Walker's determination that Proposition 8 lacked a rational basis is contrary to both well-established precedent and democratic principles."