Randy "Rasheed" Wilson entered the plea in Mobile in a deal with federal prosecutors, who agreed to seek a maximum prison sentence of 15 years, Al.com said.
Defense attorney Dom Soto told reporters Wilson did not feel he could beat a conviction, which carried a risk of 35 years in prison.
"It was his decision to plea," Soto said. "I still maintain this is a free-speech case but I have much more confidence in the jury system than he does."
Prosecutors said Wilson was secretly recorded by the FBI making inflammatory statements on hundreds of hours of tape. The plea document filed with the court included Wilson saying on tape: "It's not a small war anymore. It's either, we're going to kill them and defeat them, or they're going to kill us and defeat us."
Al.com said Wilson, who was born in Alabama, had agreed to testify against co-defendant Mohammad Abdul Rahman Abukhdair, who is scheduled to stand trial in August.