Amnesty's Turkey chair Taner Kilic was detained in June on accusations of membership of the organisation led by the alleged mastermind of last year's failed coup, US-based Muslim cleric Fethullah Gulen.
Amnesty said in a statement that Kilic, who has spent almost six months under arrest, was ordered in the latest trial hearing to remain in detention despite statements from lawyers and a witness who "demolished the prosecution's arguments".
"The court's decision to ignore this evidence and continue his detention flies in the face of reason. It is yet another opportunity missed to correct a gross injustice," said John Dalhuisen, director of Amnesty International's Europe and Central Asia programme.
His case was merged last month with those of 10 other rights activists including Amnesty's Turkey director Idil Eser, who were detained in July on contested terror charges after holding a workshop on an island off Istanbul.
More From This Section
Prosecutors accuse Kilic of having prior knowledge of preparations for the Istanbul workshop.
Eser, German activist Peter Steudtner, Swedish colleague Ali Gharavi and five others were freed last month for the duration of the trial where they are accused of "aiding" an armed terror group.
The defendants are suspected of links to Gulen and other outlawed organisations including the Kurdistan Workers' Party (PKK), which has waged an insurgency in Turkey since 1984, and the far-left Revolutionary People's Liberation Party-Front (DHKP-C).
President Recep Tayyip Erdogan in July said the activists had been taken into custody after a tip-off that they were working against the government, comparing them to those involved in the coup bid.
Ankara accuses Gulen of ordering last year's attempted overthrow of Erdogan but the preacher denies the charges.