2013 season

On April 26, 2013, Glennon was drafted by the Tampa Bay Buccaneers in the third round (73rd overall pick) of the 2013 NFL draft.^[19] He was the third quarterback taken in the draft behind EJ Manuel (16th pick) and Geno Smith (39th pick).^[20] On June 12, 2013, Glennon signed a four-year contract with the Buccaneers worth $3,104,063, with a $637,500 signing bonus.^[21]

On September 25, 2013, the Bucs announced that Glennon would start in Week 4 over Josh Freeman, after the team started the season 0–3.^[22] Glennon threw his first career touchdown to wide receiver Mike Williams, but threw a critical late interception to Arizona Cardinals cornerback Patrick Peterson and lost 13–10.^[23]

Despite Glennon throwing seven touchdowns to one interception and achieving a passer rating over 90 in the next four games, the Buccaneers lost their next four games to drop to 0–8. He earned his first win as an NFL starter the next week against the Miami Dolphins, though he had just 139 passing yards and an interception.^[24] He won his next two games as well with a combined passer rating of 142.0 and completing 77% of his passes. He went 1–4 the rest of the season, without reaching a passer rating of 80.^[25]

The Buccaneers finished their season with a 4–12 record and 4–9 with Glennon at quarterback.^[26] He finished his rookie campaign with an 83.9 passer rating, completing 59.4 percent of his passes (247 of 416) for 2,608 passing yards, 19 touchdowns, and 9 interceptions.^[25] Glennon was named to the Pro Football Writer's Association's All-Rookie Team for his strong first year performance, becoming the second Buccaneers quarterback to receive this award, joining Doug Williams (1978).^[27]

2014 season

New head coach Lovie Smith quickly announced that newly signed journeyman Josh McCown would be the starting quarterback for the upcoming 2014 season, leaving Glennon's future in Tampa Bay in question.^[28] On May 10, Smith stated while McCown would be the starter, Glennon was the Bucs' "quarterback of the future". That same day, Rick Stroud of the Tampa Bay Times reported that the Bucs turned down at least six trade offers for Glennon during the 2014 NFL draft.^[29]

Glennon made his 2014 season debut Week 3 on Thursday Night Football against the Atlanta Falcons. Glennon entered the game near the end of the first half, after McCown suffered a thumb injury. At the time of Glennon's entry into the game, the Buccaneers already trailed 35–0. Glennon completed 17 of 24 passes for 121 yards, 1 touchdown, and no interceptions.^[30] The Bucs lost the game 56–14.^[31]

Glennon started the next five games in relief of the injured McCown. In his first game as starter, Glennon recorded his first career 300 yard game and the Bucs first season win, 27–24 over the Pittsburgh Steelers.^[32] However, the Bucs lost their next four games (two in overtime), and Smith brought McCown back as starter in Week 10.

The Buccaneers finished the season with a league worst 2–14 record, and 1–4 with Glennon as the quarterback.^[33] Glennon finished his second season campaign with an 83.3 passer rating. He completed 57.6 percent of his passes (117 of 203) for 1,417 passing yards, and threw 10 touchdowns and 6 interceptions.^[34]

2015 season

Glennon did not see any playing time during the 2015 season. He was the backup to the number one overall pick in the 2015 NFL draft, Jameis Winston.^[35]

2016 season

In 2016, Glennon was again the backup to Winston. After Winston suffered a knee injury midway through the fourth quarter of the Week 9 game against the Falcons, Glennon entered and completed 10 of 11 passes for 75 yards and a touchdown.^[36] Glennon also appeared in the Week 10 game against the Chicago Bears but did not record any statistics in three plays.^[37]