The London-based human rights watchdog said a verdict in Rajab's trial was expected on Wednesday, insisting that any conviction would be a "terrible injustice".
"It would only be further proof that respect for the right to freedom of expression in Bahrain is under attack," said Amnesty's Hassiba Hadj Sahraoui.
Rajab, a member of Bahrain's Shiite majority that has demonstrated against the royal family since 2011, was arrested on October 1 after posting comments on Twitter about the interior and defence ministries.
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In one of the tweets that were deemed offensive, Rajab said Bahrainis allegedly joining Islamist extremists in Syria's war were originally members of the Sunni-ruled kingdom's security forces.
"Many #Bahrain men who joined #terrorism & #ISIS came from security institutions and those institutions were the first ideological incubator," he wrote, using one acronym for the Islamic State jihadist group.
Advocacy groups have called for the release of Rajab, who heads the Bahrain Centre for Human Rights.
He was freed in May after serving two years in jail for taking part in unauthorised protests.
Rajab had led anti-government marches following a crackdown on Shiite-led demonstrations against the Al-Khalifa ruling family in March 2011.