The Netherlands international only made his move to the North London club last summer, but failed to make an impact in the Premier League.
The 23-year-old forward joined for £18.5 million from AZ Alkmaar, where he scored 27 Eredivisie goals in 34 appearances and won the Dutch “Talent of the Year” award.
Janssen was unable to live up to his hype last season, only making seven Premier League starts, as he primarily played the role as back-up for golden boot winner Harry Kane.
The deadline day signing of Fernando Llorente and the omission of the Dutchman from the club’s Champions League squad ultimately led to a decision to look for first team opportunities elsewhere.
Janssen took to Instagram to thank the Tottenham fans for their “ongoing support” and confirmed his move to Fenerbahce.
The Turkish transfer window is still open until Friday night, which allowed Janssen to complete his loan move.