A general prosecutor, Hussein al-Buali, said Rajab was charged with "publicly insulting government institutions," after the defence and interior ministries lodged complaints over his comments on Twitter.
Shiite Rajab, who heads the Bahrain Centre for Human Rights, was released in May after serving two years in jail for participating in unauthorised protests.
But he was arrested on October 1 over his tweets.
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In one tweet, Rajab charged that Bahrainis allegedly joining Islamist extremists in Syria 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 an acronym for the Islamic State jihadist group.
Rajab was a leader of Shiite-led demonstrations against the ruling Al-Khalifa family in February 2011 that were crushed a month later.
Tiny but strategic Bahrain, home to the US Navy's Fifth Fleet, remains deeply divided and sporadic protests still often spiral into clashes with police.