Former Arsenal captain Gilberto ended his six-year spell with Arsene Wenger's side after passing a medical in Athens. The 31-year-old has signed a three-year deal worth an estimated £6 million (7.5 million euros), making him the highest-paid player in Greek football.
However, Gilberto, who was greeted by over 1,000 fans when he arrived in Greece, insisted he is motivated by the chance to win titles and not the size of his bank balance.
"I'm hungry to win more titles. I came to Greece because I have big ambitions, it's very important for me to be able to continue to give my best," he said. "(Panathinaikos manager) Henk ten Cate is a coach I know from his time at Chelsea and I'm looking forward to working with him.
"The people here at Panathinaikos made the decision to come here very easy. I'm sure we will qualify through to the group stages of the Champions League and I'm exited about being a Panathinaikos player."
Gilberto, a FIFA World Cup™ winner in 2002, was a member of the "invincible" Arsenal team that won the 2004 Premier League title after going unbeaten through the entire campaign.
He made 244 appearances for Arsenal, scoring 24 goals, and also won the FA Cup in 2003 and 2005.
"Arsenal can confirm that Gilberto has joined Greek side Panathinaikos," a statement on the club's website read. "Everyone at Arsenal would like to thank Gilberto for his huge contribution to the club both on and off the pitch, and wish him the very best for the future."