"It's time for change, and for a new chapter. Sunday's final in Berlin against Spain was my final game as England manager," Southgate said in a statement.
Names immediately bandied around as 53-year-old Southgate's successor are Newcastle manager Eddie Howe, and two former Chelsea handlers, Graham Potter and Mauricio Pochettino.
Football Association CEO Mark Bullingham said Southgate had "made the impossible job possible".
Southgate took over in 2016 when England's fortunes were at a low ebb, having exited the Euros after defeat to Iceland and then Sam Allardyce had to resign over a scandal.
In the last four tournaments Southgate led England to three semi-finals and two finals.
Prior to his appointment, the Three Lions had reached just three major tournament semi-finals and one final in their history, when they won the 1966 World Cup.
But despite changing the fortunes of the England team, Southgate was unable to land a first trophy for 58 years as England lost 2-1 to Spain on Sunday.
"Gareth has made the impossible job possible and laid strong foundations for future success," said Bullingham in a statement.
"In the 25 tournaments post 1966 before Gareth took charge, we had won seven knockout games.
"In his four tournaments we have won nine. So, in his eight years, he has won more games that really matter than in the previous 50 years.
"And of course, we have had strong tournament performances throughout their tenure.
"We came so close to winning the Euros in London and securing the first trophy for our men's team for over 50 years -– and came so close again in Berlin on Sunday."
Southgate, who is also credited for restoring pride and lustre to the team as well as turning the players into role models off the pitch, said the job had been a dream of a lifetime for him.
"As a proud Englishman, it has been the honour of my life to play for England and to manage England," he said.
"It has meant everything to me, and I have given it my all."
