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.
Rajab had led anti-government marches following a crackdown on demonstrations against the Al-Khalifa ruling family in March 2011, AFP reported.