His former team in the Dominican, Leones del Escogido, said he died of complications from diabetes.
Born on Dec. 22, 1938, in Haina, Dominican Republic, Alou was among the first wave of big leaguers from his country after Ozzie Virgil and Felipe Alou, Matty's older brother, opened the door in the late 1950s.
Alou was a platoon player for six unremarkable seasons (1960-65) in San Francisco, his finest moment coming in 1962, when his pinch-hit bunt single in the final game of a three-game tie-breaking playoff series against the Dodgers began the rally that won the game and the NL pennant for the Giants.
Matty, Felipe and their younger brother, Jesus, made history on Sept. 15, 1963, when they became the only trio of brothers in baseball history to play in the same outfield, for the Giants in Pittsburgh's Forbes Field.
The three combined for 5,094 career hits, beating out Joe, Dom and Vince DiMaggio for the best three-brother total of all time.
"He was the smallest of the three, but he was the fastest and was more of a line-drive hitter," said former Dodgers Manager Tommy Lasorda, who managed the Alou brothers in the Dominican winter league in 1975.
"He didn't have much power, but like Maury Wills, he got the most out of his abilities. If guys like that tried to hit the ball out of the park, it would mess them up."
Alou's career blossomed in 1966 after he was traded to Pittsburgh, where Pirates Manager Harry Walker helped the left-handed-hitting Alou revamp his swing.
Alou choked up on a heavier bat and often hit off his front foot, flicking his wrists at pitches and flaring many balls to left field.
His unorthodox style wasn't pretty. Hall of Famer Ted Williams, among the greatest hitters in baseball history, told the Sporting News that Alou "violates every hitting principle I ever taught," and Hall of Fame pitcher Steve Carlton once called Alou "the worst .300 hitter I've ever seen."
But it worked. Alou hit .342 with 18 doubles, nine triples and 86 runs in 1966 and edged out his brother, Felipe, for the batting title.
Flanked that season by right fielder Roberto Clemente, who hit .317 with 29 homers and 119 runs batted in, and left fielder Willie Stargell, who hit .315 with 33 homers and 102 RBIs, Alou helped form one of the most productive outfields in Pirates history.
In a decade dominated by pitching, Alou had a .327 average in five years at Pittsburgh, batting .332 in 1968, .331 with a league-leading 231 hits and 41 doubles in 1969 and making the All-Star team both years.
Alou spent his final four years in the big leagues playing for St. Louis, Oakland, the New York Yankees and San Diego, and he was a member of Oakland's 1974 World Series-winning team.
He closed his playing career with three seasons in Japan and later worked in the Giants' front office.
Alou's nephew Moises also played in the majors, including two seasons with the Giants when his father Felipe was manager.
"It's a family legacy," Felipe said of his brothers and son in 2005. "The Alou legacy is a legacy in itself. I see all of us together being a force going through this game, and still going."
According to the Associated Press, Alou is survived by his wife, Maria Teresa, sons Mateo Jr. and Matias, daughter Teresa, four grandchildren and five siblings -- brothers Felipe, Jesus and Jose, and sisters Zula and Virginia.