After reports surfaced Tuesday about his retirement, Ortiz posted a video on The Players' Tribune website.
"I thought a lot about it," Ortiz said Wednesday in the two-minute, 26-second video. "Every single one of us, athletes-wise, we run out of time at some point. Life is based on different chapters and I think I'm ready to experience the next one in my life."
Next year will be Ortiz's 20th in the major leagues. He has spent the past 13 years with the Red Sox after six years with the Minnesota Twins at the start of his career.
"I would like people to remember me as a guy that was just part of the family, you know, a guy that was trying to do the best, not just on the field but with everyone around him," Ortiz said. "Baseball is not just based on putting up numbers. This is our second family. Whoever is around you on a daily basis is like a second family, and I always had good thoughts for everyone around me.
"Baseball, besides God, it just helped flip my whole life over, not just mine, my whole family, you know what I'm saying, because I see how people struggle out there. I struggled before and I know how hard it is to make it to the top. It's something you've got to thank God every day for."
Ortiz, a nine-time All-Star and a six-time Silver Slugger Award winner, finished last season with a .273 average, 108 RBIs and 37 home runs.
The popular, Dominican-born Ortiz joined the 500-home run club this past season and finished the year with 503 career homers, 1,641 RBIs and a .284 lifetime batting average. He has hit 445 of his homers since joining the Red Sox in 2003. His 447 home runs as a designated hitter are the most in history.
Ortiz stands third in franchise history in home runs behind Ted Williams (521) and Carl Yastrzemski (452). He ranks 27th on the all-time home run list, just one away from tying Hall of Famer Eddie Murray.
"I'm really proud of what I had accomplished through the years," Ortiz said. "I'm very thankful for having fans like you guys who have supported me through my career. I wish I could play another 40 years to have you guys behind me, but it doesn't work that way."
Ortiz led the Red Sox to three World Series championships in 2004, 2007 and 2013. He owns the best World Series batting average (.455), on-base percentage (.576) and slugging percentage (.795) for players in the Fall Classic. He was named MVP of the 2013 World Series.
"It is difficult to adequately convey what David Ortiz has meant to the Boston Red Sox," Red Sox owner John Henry said in a statement. "For his teammates, he has been the one constant force underpinning what it means to play for this organization and making it fun. For the fans, he has been the one consistent force behind three world championships, lifting all of us on his broad shoulders exactly when we needed it. For the community, he has been the hero providing leadership off the field in ways that consistently make a difference often completely unseen. And for those of us who have had the honor of knowing him all these years, he has been exactly what you hope to see in a man who has been the face of this organization.
"As he concludes his illustrious career in this, his final season, we look forward to joining everyone in the game of baseball in showing him just how much Big Papi has meant to all of us."
"He's given all of our fans so much reason to cheer," Brady said Wednesday. "He's been an incredible player. It'll be sad to see him go. I've got a lot of respect for him and the way that he's always brought a great leadership to his team. He's been a great example."
Ortiz's base salary for the 2016 season is $11 million, which became guaranteed in August when he reached 425 plate appearances in 2015.
"David Ortiz transcends numbers, home runs, and wins," said Red Sox president Sam Kennedy. "He is one of the game's greatest players -- and greatest champions -- and he has been there for the city of Boston through thick and thin every step of the way. He has been a pillar of our team and a pillar of our city. We look forward to a final season of raising our heads to the sky looking for his long ball, and watching him point to the heavens when he arrives home one final time."