In a statement, Chelsea said “results and performances have not met the club’s expectations, leaving the club mid-table without any clear path to sustained improvement”.
Lampard said he was “disappointed not to have had the time this season to take the club forward and bring it to the next level”.
Tuchel, who left Paris Saint-Germain in December and previously managed Borussia Dortmund, will sign an 18-month contract to be Chelsea’s next head coach, with an option to extend for a further 12 months.
Plans are underway to appoint him before the game against Wolves on Wednesday. Tuchel is not exempt from quarantine protocols, but the FA will allow him to attend Chelsea’s training ground and matches if he tests negative to fly into the UK and then tests negative to enter the team’s bubble.
The German, who is expected to travel to the UK in the coming hours, is the only candidate for the role.
Lampard, who is the club’s all-time leading goalscorer, enjoyed a good first season as Blues boss, taking them to the FA Cup final and securing qualification for the Champions League after giving first-team roles to a number of academy graduates.
But after being given more than £200m to spend on new players in the summer, Chelsea find themselves ninth in the Premier League after 19 games, 11 points behind leaders Manchester United and five points off Liverpool in fourth.
“When I took on this role I understood the challenges that lay ahead in a difficult time for the football club,” Lampard said in a statement to the League Managers Association.
“I am proud of the achievements that we made, and I am proud of the academy players that have made their step into the first team and performed so well. They are the future of the club.
“I am disappointed not to have had the time this season to take the club forward and bring it to the next level.”