It issued a statement on Monday, stressing that Salman is a prisoner of conscience detained solely for peacefully expressing his views.
It said Salman’s detention and trial is “a blatant violation of his right to freedom of expression”.
The rights group went on o say that the move shows “the Bahraini authorities continued attempt to suppress political opposition and silence critical voices.”
Sheikh Salman, who is secretary general of Bahrain’s al-Wefaq National Islamic Society, was arrested in late December 2014 over alleged anti-regime incitements. Manama accuses him of seeking regime change and collaborating with foreign powers.
His arrest has triggered condemnation inside and outside Bahrain, with leaders, governments and international organizations across the world calling for his immediate release.
Bahrain, which is home to the US Navy's 5th Fleet, has been witnessing almost daily protests against the ruling Al Khalifa dynasty since early 2011, with Manama using heavy-handed measures in an attempt to crush the demonstrations.
Scores of Bahrainis have been killed and hundreds of others injured and arrested in the ongoing crackdown on the peaceful demonstrations.