He was diagnosed with dementia in March 2019 and had not made any public appearances since then.
"We are heartbroken to share that our beloved husband and father has passed away," his family said in a statement. "We send our love out to his fans, as we mourn his loss with you."
He is survived by his wife, Nancy, and daughters Sarah and Anne.
Seaver won 311 games during his 20-year career in the majors, compiling a 2.86 ERA while ranking sixth all-time with 3,640 strikeouts.
His 106 wins above replacement ranks seventh in MLB history among pitchers, per Baseball Reference.
The right-hander was best known for his career with the New York Mets. He won the NL Rookie of the Year in 1967 and added three Cy Young Awards with three ERA titles in his 12 seasons with the team. He was named an All-Star 10 times with the Mets and twice with the Cincinnati Reds.
He also led the league in strikeouts five times with New York and pitched a no-hitter in 1978, his first full season with Cincinnati after being traded.
Seaver was a leader for the 1969 Amazin' Mets, a squad that shockingly won the World Series after seven straight losing seasons for the franchise. The ace finished second in the MVP voting that season behind only Willie McCovey.
After ending his career with the Chicago White Sox and Boston Red Sox, he retired in 1986 and was inducted into the Hall of Fame in 1992.
Following his playing career, Seaver worked as a broadcaster for both the Mets and New York Yankees before opening a vineyard in his native California.