He retired from politics in 2021 after suffering a crushing defeat in a presidential election.
After six years in office he left the country heavily in debt and with a precarious economy.
But Mr Lungu is now aiming to capitalise on growing dissatisfaction with his successor, Hakainde Hichilema.
His return had long been anticipated but this was the first formal announcement.
“I am ready to fight from the front, not from the rear, in defence of democracy. Those who are ready for this fight, please come along with me, I am ready for anything,” Mr Lungu told his backers on Saturday.He was speaking at a commemoration service to former president and leader of his Patriotic Front (PF) party, Michael Sata, who died in office in 2014.
Since Mr Lungu lost power two years ago, there has been a wrangle within the PF over who should lead the party that has ended up in court. The entrance of Mr Lungu could well heighten tensions and prolong the legal battle, as there is currently another person, Miles Sampa, who claims to have been elected as PF president at a recent meeting.
Some, including the former president, have accused the government of stoking divisions, which it denies.
Mr Lungu “enjoys a constitutional and democratic right to be able to participate in the political sphere of our country and if he so wishes he is welcome back in the political arena”, Information Minister Cornelius Mweetwa told Zambian public broadcaster ZNBC.
Mr Hichilema won the 2021 election, his sixth run at the presidency, with 59% of the vote on the hope that he would tackle the country’s economic problems.
But those high expectations are taking a long time to be met and patience is running thin.
The president has succeeded in negotiating a bailout agreement for the copper-rich country with the International Monetary Fund. He has also been able to restructure loan agreements with Chinese and other creditors.
Mr Hichilema argues that he inherited a country in deep economic trouble in which the all-important mining sector was struggling.
But the ordinary consumer is facing very high prices for staple foods and fuel, making life difficult for many and leading to frustration with the political leadership.
The government was also criticised this week for a “growing intolerance for dissent” by 13 leading civil society organisations. In a joint statement, they said there was a “shrinking space for freedom of expression and assembly in the country”.
The presidency has argued in the past that it respects human rights and does not interfere in the operations of the police.
Last month, Mr Lungu was warned against jogging in public, with police describing his weekly workouts as “political activism”.
His Saturday runs with ordinary members of the public and PF supporters were been attracting a lot of attention. The former president was told to seek police approval for future jogging events.