Unitas died of a heart attack, Baltimore Ravens spokesman Chad Steele said. Steele had no other details.
Unitas underwent emergency triple-bypass surgery in March 1993 after a heart attack.
He was the first to throw for 40,000 yards in his career and now ranks seventh, surpassed by a group of quarterbacks who played in an era when the rules made passing easier. Unitas retired after the 1973 season holding 22 NFL records, among them marks for most passes attempted and completed, most yards gained passing, most touchdown passes and most seasons leading the league in TD passes.
Unitas completed 2,830 of 5,186 passes for 40,239 yards and 290 touchdowns. He completed at least one touchdown pass in 47 straight games, a record that no one has come close to matching since it was set from 1956-60.
Unitas was Most Valuable Player three times and played in 10 Pro Bowls. He led Baltimore to the NFL championship in 1958 and 1959 and the Super Bowl in 1970. He was inducted into the football Hall of Fame in 1979.
On the NFL's 50th anniversary in 1969, Unitas was voted the greatest quarterback of all time. He also was selected at quarterback for the NFL's All-Time team in 2000 by the 36 Pro Football Hall of Fame voters.
"Johnny Unitas is the greatest quarterback ever to play the game, better than I was, better than Sammy Baugh, better than anyone," Sid Luckman, the great Chicago Bears quarterback of the 1940s, once said.
Unitas was one of the few quarterbacks who called his own plays, an ability traced to his knack for reading an opponent's defense and spotting a weakness, then calling a play to take advantage.
John Mackey, the Colts' tight end during the Unitas years, once said of his teammate, "It's like being in a huddle with God."
The long list of accomplishments was quite a reversal of fortune for a player who hitchhiked home from his first NFL training camp after the Pittsburgh Steelers cut him in 1955. He spent that season playing semipro football on rock- and glass-covered fields in Pittsburgh for $6 a game and working as a piledriver at a construction site.
The Colts signed him the following season after getting tipped to his ability in a most unusual way.
"Unitas was signed after we received a letter from a fan telling us there was a player in Bloomington deserving a chance," former Colts coach Weeb Ewbank recalled a few years later. "I always accused Johnny of writing it."
Unitas became a backup quarterback and made his debut in the fourth game of the 1956 season. His first pass was intercepted and returned for a touchdown. It got worse as Unitas fumbled on his next two possessions.
Fortunately, however, the Colts' other backup had opted for law school and Unitas was able to start the next game, and Baltimore beat the Green Bay Packers 28-21. A week later, the Colts upset the Cleveland Browns, and Unitas had earned himself a job.
Unitas didn't really look like a football player. At 6-foot-1, just under 200 pounds, his body was that of an everyday person — except for the scars, bumps and bruises.
His most noticeable malady was a curved right arm, evidence of the thousands of passes he threw. His worst injury was a torn Achilles' tendon, but he also had broken ribs, a punctured lung and knee injuries.
Unitas' brightest moment probably came in the 1958 championship game against the New York Giants, a match that was called "the greatest football game ever played" for years afterward.
With 90 seconds left, Unitas completed four passes, taking the Colts to the 20-yard line to tie the game on a field goal. He then engineered an 80-yard drive for the winning touchdown.
"The drama came from the championship setting rather than the game itself, until we came down to tie it in the final seconds. And then it became the first playoff ever to go to sudden death, and you can't have much more drama than that," Unitas recalled.
The following year, Baltimore beat the Giants 31-16 in the championship game. Unitas ran for one touchdown, and passed for two others, completing 18 of 29 passes, good for 264 yards. For the season he set an NFL record by throwing 32 touchdown passes, and was named the league's outstanding player.
His Super Bowl victory came in 1971, a 16-13 victory over Dallas in which he played sparingly. He also played in the 1969 Super Bowl, a shocking 16-7 loss to Joe Namath and the New York Jets.
Unitas' enormous talent and ability, combined with his penchant for taking command in the huddle, caused some players to view him as overly cocky and arrogant.
Unitas called it confidence.
"There's a big difference between confidence and conceit. To me, conceit is bragging about yourself. Being confident means you believe you can get the job done, but you know you can't get your job done unless you also have the confidence that the other guys are going to get their jobs done too. Without them, I'm nothing," he said.
Some of that confidence was apparent in his freshman year of college at Louisville. He threw for more than 2,000 yards and 21 touchdowns in his first two years, earning the nickname "Mr. Football" from local sports writers.