Tottenham Hotspur
On 12 July 2016, Janssen joined Premier League side Tottenham Hotspur for £17 million (€20 million) on a four-year contract. Manager Mauricio Pochettino had seen Janssen during a Netherlands friendly with the Republic of Ireland, saying Janssen "ticked all the boxes we needed". With his signing, he became the most expensive player sold by AZ Alkmaar in their football history.
Janssen played in three friendlies for Tottenham before the 2016–17 season, providing a goal and an assist in the final game against Inter Milan. Janssen made his Premier League debut on 13 August, coming on as a substitute for Eric Dier against Everton. He made his first start the next week at home to Crystal Palace, playing at forward the full 90 minutes. Following an ankle injury to star Harry Kane on 18 September, Janssen was thrust into a larger role for the club, with Pochettino stating "this is a big opportunity for him to show what he can do". He scored his first goal for the club on 21 September 2016, converting a penalty in a 5–0 victory in the EFL Cup against Gillingham. His first Premier League goal, another penalty, was on 29 October in a 1–1 draw with Leicester City. He scored his first goal from open play for Tottenham on 12 March 2017, scoring the fifth goal in a 6–0 FA Cup Quarter Final win over Millwall. On 4 September 2017, it was confirmed that Janssen had been omitted from Spurs' Champions League squad for the 2017–18 campaign.
On 8 September 2017, Janssen joined Süper Lig club Fenerbahçe for the season. He scored his first goal for the club in a 4–1 win over Alanyaspor, and a second in the Istanbul derby against Beşiktaş that resulted in a 2–1 win. However, he was injured against Bursaspor in early December, and was out of action for a few months.
In summer 2018 Janssen returned to Tottenham but at the beginning of the 2018–19 season he was not issued a number. Spurs reported on their website that Janssen had surgery for a foot injury and would be in recovery with the club's medical staff. In January 2019, Pochettino publicly said that Janssen was not in his plans, despite Kane being injured and Son Heung-min being away at the AFC Asian Cup; he was subject to interest from Major League Soccer and teams from China and Europe, but Tottenham would not sell for under £15 million. After being out for most of the season Janssen made a substitute appearance and a return to first team football for Spurs against Brighton on 23 April 2019.
