Schröder travelled to Turkey a week after the German general election on September 24th to speak with Erdogan about the activist, Peter Steudtner, and other German detainees in Turkey.
"I am very grateful to Gerhard Schröder for his intervention," Foreign Minister Sigmar Gabriel told news weekly Der Spiegel.
Schröder met with Chancellor Angela Merkel before travelling to Ankara, according to Der Spiegel.
"It is a first sign of detente, as the Turkish government has kept all its commitments," Gabriel said.
"Now we must continue to work for the release of the other detainees."
Steudtner, an activist and documentary filmmaker, had been arrested along with members of Amnesty International in July and held on widely disputed terror charges.
A Turkish court ordered the conditional release of eight human rights activists, including Steudtner, late Wednesday.
Ten German nationals remain in Turkish custody for what Berlin has condemned as political reasons, including prominent journalist Deniz Yucel.
Schröder, like Gabriel a member of the Social Democratic Party, was chancellor of Germany from 1998 to 2005.
He recently caused controversy - and drew fire from Merkel - for accepting a $500,000-a-year pre-tax board job with state-controlled Russian oil giant Rosneft.
The company is subject to Western sanctions over Moscow's role in the Ukraine conflict.