The Greek foreign ministry identified the aid worker as Eleni Sotiriou, the vice-president of the non-governmental organisation Doctors of the Heart.
"Mrs Sotiriou is safe and healthy at the Greek embassy building in Baghdad," the ministry said in a statement.
Sotiriou had been kidnapped outside a central Baghdad hotel on October 30.
Through contacts in Baghdad, the Greek authorities had managed to stay in indirect communication with the activist, who succeeded in evading her captors on Friday, a Greek foreign ministry source told AFP.
"We had asked media not to disclose news of the kidnapping because this would not have helped the operation," the ministry official said, adding that no ransom was paid to secure Sotiriou's release.
"There are a number of criminal gangs in Baghdad who kidnap activists and sell them back to organisations," the official said.
In Baghdad, Brigadier General Abdul Karim Khalaf of the Iraqi interior ministry told AFP that the activist would "stay in Baghdad and work in the health sector".
Baghdad has been engulfed in a wave of insurgent and sectarian violence, with several kidnappings reported daily.

Copyright AFP (Agence France-Presse), 2006