"This is a dream that I've had since I was little, and actually achieving it is really amazing," Fernandez said. "Having my family here and so much support from this amazing country, it's really fantastic."
The Marlins presented Fernandez with an American flag that had flown over the team's stadium.
The 22-year-old Fernandez arrived in the U.S. in 2008, settling in Tampa, Florida. He was a 2011 first-round pick, taken 14th overall by the Marlins, out of Tampa's Alonso High School.
He was the NL Rookie of the Year and an All-Star in 2013. He's currently on the disabled list after missing most of the 2014 season because of Tommy John surgery.
"This is one of my important accomplishments," Fernandez said. "I'm an American citizen now -- I'm one of them. I consider myself now to be free."
The new citizens sworn in represented 22 different countries. They heard taped presentations from President Barack Obama and former U.S. Secretary of State Madeleine Albright, who was born in Czechoslovakia and became a U.S. citizen in 1957.
After the swearing-in ceremony, Fernandez took to the podium for his speech.
"Today is not really about I can throw 100 miles or I could be really good," Fernandez said. "Today is really important because all of us are the same today. We just became citizens.
"I've accomplished a couple of things in my life like probably you guys have, too, and I think, in my book, this is a huge one. . Everybody here probably had to make a tough decision to come (to the U.S.), right?
"I thank this amazing country for giving me the opportunity to go to school here and learn the language and pitch in the major leagues. It's an honor to be a part of this country, and I respect it so much."
The Associated Press contributed to this report.