Philip Clarke's departure as Tesco chief executive follows a disappointing set of sales figures for the first half of this year, the company said in a statement.
The supermarket giant said "challenging" trading conditions and a weaker than expected market meant "sales and trading profit in the first half of the year are somewhat below expectations".
The company's chairman Sir Richard Broadbent paid tribute to Mr Clarke's achievements but said it was time for "a new leader with fresh perspectives and a new profile".
Mr Clarke said it was "the right moment" to hand over the reins and that he was "delighted" that Unilever executive Dave Lewis had been chosen as his replacement.
More top news
More deprived areas of England have received the biggest cuts over the last five years, according to an influential think-tank.
A suspected burglar and domestic abuser who goaded police on their own Facebook page has been taunted back after handing himself in.
Motorists are to be allowed 10 minutes' grace after a parking ticket runs out before they can receive a fine, under new rules.