The German’s arrival at Stamford Bridge follows the sacking of Frank Lampard on Monday.
Tuchel was fired as Paris Saint-Germain manager in December after two-and-a-half years at the French club, with whom he won two Ligue 1 titles, one Coupe de France and one Coupe de la Ligue.
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The 47-year-old also led PSG to last season’s Champions League final, which his side lost 1-0 to Bayern Munich.
Prior to his spell with PSG, Tuchel coached Borussia Dortmund between 2015 and 2017 and fellow Bundesliga clubs Mainz (2009-2014) and Augsburg (2007-2008). He lifted one DFB Pokal while in charge of Dortmund.
Tuchel takes over at Chelsea with the Blues 10th-placed in the Premier League on 29 points – 11 behind leaders Manchester United – at the exact midway point of their top-flight campaign.
Chelsea announced that Tuchel would take his first training session on Tuesday evening and that the German will be in the dugout for Wednesday’s Premier League fixture against Wolves.
Chelsea are still in the FA Cup – having beaten Luton 3-1 in the fourth round on Sunday, one day before former Blues midfielder Lampard was dismissed – and will take on Atletico Madrid in the last 16 of the Champions League across February and March.