He announced his decision to quit in a statement issued by the Football Association on Tuesday morning, following the team's defeat to Spain in the Euro 2024 final on Sunday night.
"As a proud Englishman, it has been the honour of my life to play for England and to manage England. It has meant everything to me and I have given it my all," Southgate said in a statement.
"But 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 took charge in 2016, following the team's ignominious exit to Iceland in that summer's Euros and the short-lived reign of Sam Allardyce.
FA chief executive Mark Bullingham paid tribute to Southgate's achievements in leading England to successive European Championship finals, as well as to the semi-finals of the 2018 World Cup in his first tournament at the helm.
Bullingham indicated the process to find Southgate's successor "is now under way" and said the FA had an "interim solution in place if needed", with the team's next matches coming in the Nations League in September.
Southgate himself backed his players to go on and "win the trophy we all dream of" in the future.
"I am so proud of them, and I hope we get behind the players and the team at St George's Park and the FA who strive every day to improve English football and understand the power football has to drive positive change," he said.
Bullingham's statement highlighted just how far England had come on the pitch under Southgate.
"In the 25 tournaments post 1966 before Gareth took charge, we had won seven knockout games. In his four tournaments we have won nine," Bullingham said.
"So, in his eight years, he has won more games that really matter than in the previous 50 years.
"Before Gareth, our longest time ranked in the top five in the world was seven months. In his tenure we have been ranked there for six years."
Bullingham also highlighted Southgate's wider impact on the team, beyond the on-field achievements.
"We look back at Gareth's tenure with huge pride - his contribution to the English game, including a significant role in player development, and in culture transformation has been unique," he said.
Current England U21 boss Lee Carsley, who led his group to Euros success last summer, would appear an obvious contender to take on the job on an interim basis, if not in the longer term.
Southgate initially succeeded Allardyce on an interim basis in 2016.
England take on the Republic of Ireland in Dublin on September 7 before facing Finland at home on September 10.
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