Campbell finished the season as Hull's top scorer, with 15 goals in 32 appearances. On 24 May 2008, he played in the Hull City team that achieved promotion to the top flight for the first time in their 104-year history. Campbell provided the assist for Dean Windass' goal in a 1–0 win over Bristol City in the Championship play-off Final at Wembley Stadium.
In July 2008, Campbell was selected for the pre-season tour of South Africa with the Manchester United first team, and scored the team's fourth goal in a 4–0 win over Kaizer Chiefs in the final of the 2008 Vodacom Challenge. He also scored the winning goal in Ole Gunnar Solskjær's testimonial match against Espanyol on 2 August 2008. After the match, United manager Alex Ferguson indicated that Campbell would remain at the club for the duration of the season, stating "Fraizer's future is here". Campbell earned his first winner's medal when he came on as a substitute in United's penalty shoot-out win over Portsmouth in the 2008 FA Community Shield. On 17 August 2008, Campbell started alongside Wayne Rooney for the opening match of the season in a 1–1 draw against Newcastle United.
On 1 September 2008, transfer deadline day, Hull City bid a club record fee of £7 million for Campbell. However, he instead signed for Tottenham Hotspur on a season-long loan, as part of the transfer of Dimitar Berbatov to Manchester United for £30.75 million. This was contradictory to Ferguson's earlier claim that Campbell would be staying at United for the season. He explained the deal saying; "Tottenham insisted that he be part of the deal and Fraizer's signed his own agreement to go there for the year so we're happy with that." He made his Tottenham debut on 18 September 2008, coming on as a 56th-minute substitute for Aaron Lennon in a UEFA Cup first-round first-leg match against Wisła Kraków. Within 15 minutes of coming on, Campbell provided Darren Bent with the assist for the winning goal. Campbell scored his first two goals for Tottenham in a 4–2 victory over Liverpool in the League Cup fourth round, as well as setting up his strike partner Roman Pavlyuchenko for the match's opening goal. On 15 November 2008, Campbell scored his first ever Premier League goal late in a 2–1 defeat to Fulham.
During his loan spell at Hull City, Campbell received his first call-up to the England under-21 team, coming on as a substitute in a match against Poland on 25 March 2008. He scored his first international goal on 18 November 2008 in a 2–0 victory over the Czech Republic Under-21s. He also scored in the second group match of the 2009 UEFA European Under-21 Championship against Spain, in a 2–0 victory on 18 June 2009. He was sent off in the semi-final of the championship against Sweden. Following his under-21 experience, Campbell's Sunderland manager Steve Bruce urged Fabio Capello to consider Campbell for promotion to the senior squad saying, "the England manager should have a look at him", also saying "As for England, why not? Why not go with up-and-coming young talent?"