By Deutsche Presse Agentur
In the statement sent worldwide from its headquarters in London, the non-governmental organisation has urged the public to take part in an "urgent action" to pressure the Thai authorities to drop charges against Sirikan Charoensiri.
Sirikan was charged on October 22 with sedition for violating the junta's ban on political gatherings of at least five people.
Sirikan has represented students and young activists from the New Democracy Movement, a group of pro-democracy activists, since June.
Sirikan is working on behalf of Thai Lawyers for Human Rights, who provide legal aid to individuals arrested, detained and prosecuted under martial law and the junta's orders.
Military courts have been used for trials of political cases deemed threatening to national security since the military took power in May 2014.
Although the trial date has yet to be set, Sirikarn will be tried in a military court if prosecuted, and if convicted, she can face up to 15 years in prison, Amnesty said.
Other rights groups have also raised the alarm about violations of human rights and freedom of expression in Thailand.
"Thailand has faced a deepening human rights crisis since the May 2014 military coup. The ruling (junta) has criminalised freedom of expression; made hundreds of arbitrary arrests; and held detainees in military detention without safeguards against abuses," Human Rights Watch said.