Coach Nikos Nioplias's side went into their penultimate Super League match six points clear of arch rivals Olympiakos Piraeus and needing to avoid defeat to secure their 20th championship.
Panathinaikos made a perfect start at the Olympic Stadium when France striker Djibril Cisse converted Sebastian Leto's headed pass in the first minute for his 29th goal of the season.
"Today is a day of joy for myself and the whole club. When you have good players your dreams can be realised," Nioplias told reporters.
"Our big target was the league title. We also want to win the cup but the championship is what gives the prestige and the opportunity to play in the Champions League."
"The title belongs to our supporters who have lived through strange times in recent years," said Panathinaikos president Nikos Pateras referring to the remarkable dominance of Olympiakos.
"I would like to thank them for the backing they have given the coach and the management from the start of the season. Without them we would not have been able to win the championship."
