Tuchel agreed an 18-month contract when he joined Chelsea in January, but has now pledged his long-term future to the club following the Champions League victory over Manchester City.
"I cannot imagine a better occasion for a contract renewal," Tuchel said. "I am grateful for the experience and very happy to stay part of the Chelsea family.
"There is far more to come and we are looking forward to our next steps with ambition and much anticipation."
Chelsea director Marina Granovskaia said: "When Thomas joined us in January there was still so much to play for domestically and in Europe. He slotted in seamlessly and immediately became an integral part of the Chelsea family.
"Returning us to the top four in the Premier League was crucial, and we could not be happier with our success in the Champions League which crowned a remarkable season at Chelsea.
"We are obviously extremely pleased therefore to retain Thomas for a further two years, and look forward to more achievements in the coming seasons."
The 47-year-old joined Chelsea after his sacking by Paris Saint-Germain in December.
He has taken charge of Chelsea for 30 games, winning 19 and losing on just five occasions.
"I will stay hungry for the next title," Tuchel said after the Champions League victory. "My desire is for more victories and to grow as a coach.
Chelsea also extended the contracts of defender Thiago Silva and striker Giroud by one year until the end of the 2021-22 season.
Silva, 36, was signed from Paris Saint-Germain on a free transfer last August on a one-year deal and featured 34 times for Chelsea across all competitions.
Chelsea added that they had exercised the option to extend Giroud's contract in April. The 34-year-old was their top scorer in the Champions League this season with six goals.