The 39-year-old Italian is currently in his second season at Sydney FC but there has been speculation he is unhappy at the club, despite being the A-league's record earner.
"We have sent him (Del Piero) a letter to ask what conditions -- including details of his salary -- he would want if he came here to play," PTT Rayong's manager Prawut Thavornsiri told AFP.
"We sent the letter two days ago and Del Piero said he needed a few days for consideration... I think it's 60 percent possibility (he will join)."
PTT is one of the biggest firms in Thailand, owning major oil and gas assets in the region.
Newly-promoted to the Thai Premier League, the club want to state their title credentials with a headline-grabbing signature before the start of the new football season next month.
Sydney FC have said they hope their gifted Italian striker sees out another season after setting the A-league alight with 14 goals in 2012-13.
"We will discuss his future when the time is right," the club's chief executive Tony Pignata was quoted as saying by Australia's AAP agency.
"The deals that his brother and agent talk about is up to them but there is always speculation surrounding a player of his calibre," he added.
Del Piero, in his prime a prolific forward blessed with magical skill and a sweet right foot, swept domestic and European honours in a trophy-laden 19 years at Italian giants Juventus.
He also starred for the national side, winning the World Cup in 2006.
A move to the relatively unknown TPL would be a coup for Thai football whose biggest foreign signing so far was ex-Liverpool great Robbie Fowler -- who endured an unremarkable spell in the twilight of his career with Maung Thong United in 2011-12.
In recent weeks former Arsenal striker Jay Simpson and one-time QPR forward Jay Bothroyd have joined top teams in the league.