After being traded as a minor league prospect by the Seattle Mariners, Varitek played his entire major league career for the Boston Red Sox. A three-time All-Star and Gold Glove Award winner at catcher, and a Silver Slugger Award winner, Varitek was part of both the 2004 World Series and 2007 World Series Championship teams. In December 2004 he was named the captain of the Red Sox, only their fourth captain since 1923.[1] He was a switch-hitter.  Varitek is one of only two players in the history of the sport to have played in the World Championship game of the Little League World Series, in the National Championship game of the College World Series, and in the Major League World Series. Varitek stands alone as the only baseball player in history to have played in the three aforementioned World Series along with playing in either the Olympic Baseball team or in the World Baseball Classic (he actually played in both the Olympics and the World Baseball Classic).  Varitek caught an MLB-record four no-hitters. Varitek played five summers in the Cape Cod Baseball League with the Hyannis Mets. He was drafted 21st overall in the first round by the Minnesota Twins in 1993, but opted to return for his senior year of college. Following graduation, Varitek signed with agent Scott Boras and was drafted by the Seattle Mariners in the first round of the 1994 amateur draft.  He was traded with Derek Lowe to the Red Sox during the 1997 season in return for reliever Heathcliff Slocumb, often cited as one of the best trades in the Red Sox's favor in recent history.  Varitek was called up for a single game on September 24, 1997, collecting a single in his only at bat.

During the 1998 season, Varitek split time with incumbent catcher Scott Hatteberg playing in 86 games. Varitek showed signs of things to come in the season, and with a strong spring training following the season, Varitek earned the starting catcher position. 1999 was a breakout year for the catcher. Varitek played in 144 games, hitting for a .269 average, with 20 home runs and 76 RBIs.  Looking forward on building more success from the year before, the 2000 season was a disappointment offensively. Prior to the 2001 season, Varitek signed a 3-year, $14.9 million contract with Boston. Varitek went on a hitting hot streak, having a .310 average at one point and on May 20, 2001, he homered three times in a single game before a broken left elbow injury sidelined the catcher for nearly the rest of the season, as Varitek dove to catch a foul ball on June 7. Varitek returned to the Red Sox lineup fulltime in the 2002 season. Despite not reaching his full offensive potential, pitchers and coaches alike began to notice how much Varitek's preparation and knowledge of the game was helping the pitchers. His study habits and extra hours of work with pitchers would soon become his defining attribute. Varitek and the Red Sox entered the 2003 season with a renewed fire to reach the playoffs after missing in the previous three years. Varitek instantly became a leader in the clubhouse which management tried to portray as working class.  The Red Sox earned a Wild Card berth and their first playoff appearance since 1999, before losing the 2003 ALCS to the New York Yankees. In 2004, Varitek compiled a career-high .296 batting average with 18 home runs and 73 RBI. During a nationally televised game on July 24, 2004, Varitek shoved his glove into the face of the Yankees' Alex Rodriguez after Rodriguez was hit by a pitch and gestured towards pitcher Bronson Arroyo, saying "Fuck you!"

"Fuck you!", causing a bench-clearing brawl. Notably, he did not take his mask off during the brawl, thereby violating one of the "unwritten rules of baseball". Though he was ejected (along with Rodriguez) from the game following the incident, the Red Sox, spurred on by the fight, came from behind to win 11–10. It is also sometimes regarded as the turning point in the Red Sox season, as they posted MLB's best record after the melee. Boston culminated the season with its first World Series championship in 86 years, after being the first team to overcome a three games to none deficit in the ALCS vs. the New York Yankees. At the end of the year, Varitek became a free agent and signed a 4-year, $40-million contract with the Red Sox. Because of his performance both on and off the field, the franchise awarded Varitek with the captaincy. After Varitek's re-signing, the Red Sox appointed him to be the third team captain in franchise history, following hall of famers Carl Yastrzemski (19691983) and Jim Rice (19861989). In 2005, Varitek won his first Gold Glove Award, his first Silver Slugger, and his second All-Star selection. On July 18, 2006, Varitek played his 991st game at catcher for the Boston Red Sox, breaking Carlton Fisk's club record. On September 19, 2006, Varitek was honored during a pre-game ceremony as the first Red Sox catcher to catch 1,000 games. He was presented with a special award by Hall of Famer Carlton Fisk, who held the Boston club record with 990 career games caught before Varitek surpassed it. Varitek caught his 1000th game on July 31 and by the evening of the ceremony had appeared in 1,009 games behind the plate. That same night, Varitek also received the 2006 Red Sox Heart and Hustle Award from the local chapter of the Major League Baseball Players Alumni Association, which is presented to a player exemplifying the values, tradition, and spirit of the game of baseball.

In 2007, Varitek and the Red Sox returned to the World Series, winning for the second time in four years. During the season, Varitek recorded his 1000th career hit. On May 19, 2008, he caught Jon Lester's no-hitter, giving him a Major League record of having caught four separate no-hitters in his career.  During the 2009 season, Varitek's numbers were similar to his dismal 2008 season. He eventually became the backup catcher when the Red Sox acquired All-Star Victor Martinez on the July 31 trade deadline. With the addition of Jarrod Saltalamacchia, Varitek usually came off the bench during the 2011 season. After the 2011 season, Varitek became a free agent once again, and was offered a minor league contract, with an invitation to spring training, by the Red Sox. On March 1, 2012, at Jet Blue Park in Fort Myers, Florida, Varitek officially announced his retirement.