Marzano, who was a first-round pick of the Red Sox in 1984, played for the Red Sox, Rangers and Mariners in a career that lasted through 1998. He was in his second season as an on-air personality at MLB.com.
"John was a beloved member of our team, a personable, terrific friend to all with whom he worked," said Bob Bowman, CEO of MLB Advanced Media. "He was an engaging, informed interviewer. His energy, knowledge of the game and comedic touch produced admirable results. We miss him dearly already."
"He passed away," Red Sox manager Terry Francona said in delivering the news to the Red Sox beat reporters prior to Saturday's game. "I got a real sketchy text [from a clubhouse attendant] earlier. I guess he fell down the stairs but had a heart attack."
John was selected by the Red Sox in the first round of the June 1984 amateur Draft and made his Major League debut with Boston in 1987. He hit six homers and drove in 44 runs in parts of six seasons with the Red Sox.
"He ... was a valuable contributor to the club on and off the field," the Red Sox said in a team statement.
"Over the last few years, John covered a number of Red Sox games in his role as a television, radio and internet journalist," the statement continued. "The same preparation and engaging personality that marked his time as a player in Boston carried over to his duties with the media. We express our heartfelt sympathies to John's family and friends at this very difficult time."
Marzano is survived by his wife, Terri, daughters Dominique and Danielle, and two grandchildren.
MLB.com will immediately establish an internship program in memory of Marzano, who in addition to being drafted 14th overall in 1984 earned a spot on Team USA for the Olympics.
"Words cannot describe how sorry we are to have lost John's inspiring presence, but rest assured, his spirit and legacy will always influence us," said Dinn Mann, MLB.com's EVP of Content. "Our prayers go out to his family, his friends and his fans. John simply gave his all every time he went about doing his job and living his life, always grateful, always striving."
A native of South Philadelphia and born on Valentine's Day in 1963, Marzano, a member of the Temple University Hall of Fame, showed tremendous work ethic and determination as a ballplayer and broadcaster.
Marzano played 10 seasons as a catcher in the Majors. The Red Sox weren't the first team to draft him. The Twins picked him in the third round in 1981 out of high school, but
Marzano elected to go to college. Marzano reached the big leagues within three years after starring at Temple, playing for Boston from 1987-1992.
He was in the Cleveland and Philadelphia farm systems for two years, then, in 1995, appeared in two games for the Texas Rangers. In 1996, he earned a spot on Lou Piniella's Seattle roster and spent three years with the Mariners.