Tahmooressi, a U.S. Marine, was arrested two months ago after he said he mistakenly took three guns across the border into Mexico. He said he had no intention of crossing the border and was originally told he could turn around to return to the United States before he was arrested.
Tahmooressi, 25, fired his attorney, Alejandro Osuna, before appearing in court Wednesday. He said Osuna told him to lie to authorities regarding the case.
"He now has a new lawyer who we have more confidence in," the Marine's mother, Jill Tahmooressi, said. "Justice will prevail, but not because of the truth from the other side."
Osuna said he didn't tell Tahmooressi to lie.
"We've had our differences about how things happened," Osuna said.
"Andrew is innocent and regardless of who his attorney is, his innocence will come out," he added.
Tahmooressi is next scheduled to appear in court June 4.