Atapattu, who was not picked in the original touring squad, was brought in as the 17th member on Thursday following the intervention of the country's sports minister Gamini Lokuge.
"Marvan attended practice this morning", team manager Sriyan Samararatne told AFP.
The 36-year-old will be the fourth opening batsman in the touring party alongside Sanath Jayasuriya, Upul Tharanga and Michael Vandort for the two Test matches next month.
Atapattu, who snubbed the national selectors after the World Cup earlier this year, had asked to be released from his annual contract with Sri Lanka Cricket (SLC) during the row.
Even though SLC had granted his wish, Lokuge stepped in to defuse the crisis following talks with officials and the selectors headed by former Test fast bowler Asantha de Mel.
Atapattu met SLC chief executive Duleep Mendis on Thursday and agreed to rejoin the national team as Sri Lanka bid for their first Test win in Australia.
"I met Duleep and we spoke about the terms and conditions under which I will be playing," said Atapattu. "I was satisfied with them and I told him that I was available for Australia."
Sri Lankan captain Mahela Jayawardene said he welcomed Atapattu's selection.
"We welcome Marvan into the team," said Jayawardene. "He is experienced and has done so well for Sri Lanka that you always welcome a guy like that.
"It all depends on how keen and committed he is to the team. That is the most important thing."
Sri Lanka leave for Australia next Tuesday and the first Test starts in Brisbane on November 8.
Atapattu's return to the national team marks the end of a six-month ordeal that began at the World Cup, where he was not selected to play in any of Sri Lanka's 11 matches despite being in the squad.
He was also overlooked for the one-day series against Pakistan in Abu Dhabi in May and he pulled out of the three-Test home series against Bangladesh in June, saying he was not mentally prepared to play Test cricket.
Atapattu played the last of his 88 Tests against India in Ahmedabad in December 2005.
He has scored 5,330 Test runs at an average of 38.90, with 16 centuries.