The London-based non-governmental organization has launched an online campaign through which sends an email to the Bahraini King via the regime's Ministry of Foreign Affairs.
The email will call on king Hamad "to free Nabeel - and stop the Bahraini authorities' crackdown on the brave people who speak out for human rights," the leading rights groups says on its campaign page.
Amnesty International describes Nabeel Rajab as a "Brave Bahraini voice and prominent human rights defender," who "Has been in detention since 2016. A prisoner of conscience, he is serving a two-year sentence for TV interviews he gave in 2015 and 2016".
Amnesty underlines the importance of such online actions, which earlier contributed to the release of Ebtisam al Saegh, another human rights defender in Bahrain.
Stating that Nabeel’s verdict is imminent on February 21, the organization said, “We must mobilize quickly and effectively, before it is too late.”
It said Rajab, who has been in detention since 2016, “spent an excruciating nine months in solitary confinement, which amounts to torture and ill-treatment.”
Last month, Bahrain’s Court of Cassation, which has the ultimate say in the case of appeals in the country, upheld a two-year jail sentence issued for the 52-year-old president of the Bahrain Center for Human Rights.
Rajab has faced two trials, one concerning his criticism of the ruling regime in alleged interviews he has given and statements made to foreign media, and another having to do with his critical tweets.
Rajab was first detained on June 13, 2016 for the tweets. He will face a new hearing over the case on February 21.
If found guilty, Rajab would face up to 15 years in jail, according to Amnesty.
Manama considers Rajab one of the leaders of the protests which the regime has been facing since 2011.
Scores of people have been killed and hundreds arrested during the crackdown with the help of Saudi and UAE troops.