The 33-year-old right-hander helped pitch the Kansas City Royals to the World Series title in 2015 as he went 13-9 with a 3.55 ERA. He slipped to 10-11 with 5.37 ERA with the Royals this year.
He assured everyone Friday that he is now rested and ready to go after four consecutive seasons with more than 30 starts.
"I pitched a lot of innings the year before," Volquez said in Spanish, referring to the 200 1/3 innings he threw in 2015. "By June and July (of 2016), I was tired."
Volquez joins a rotation shaken by the loss of ace Jose Fernandez, who died in a boating accident a week before the end of the regular season in September.
"It's hard to believe," Volquez said. "I was with the Royals when it happened. A lot of us were crying.
"Jose's death was a sad moment for MLB. He was a great pitcher, and he was only 24. But I don't want to be Jose. I want to be me."
An All-Star in 2008 with Cincinnati, Volquez has an 89-79 record in 12 major league seasons. He became a free agent this month when he declined his mutual option with the Royals.
Volquez said he has lived in Miami for three years, and he told his agent that becoming a member of the Marlins was his top priority in free agency.
Marlins general manager Michael Hill said he was "excited" to get Volquez.
"He's a World Series champion and an All-Star," Hill said. "He will fit in perfectly with our inventory. But we are not done yet. We want to add to our pitching depth."