Rajab's case appears to relate to tweets authorities allege he sent last year suggesting security forces had tortured detainees and accusing the government of complicity in killing civilians as part of a Saudi-led war in Yemen.
He was arrested earlier last month as part of what Bahraini rights groups say is an escalating crackdown by the Sunni Muslim-led government on the island's majority Shi'ites.
Government officials have not commented publicly on the case but deny systematic abuses of human rights and have accused the opposition of stirring sectarian hatred in the kingdom and serving the interests of their rival, Shi'ite Iran.
"We remind the Bahraini government of its obligation to preserve the right to free expression," the campaign groups, including Human Rights First and Physicians for Human Rights, said in a statement.
"We reiterate repeated calls by United Nations officials, and others in the international community, to immediately release Rajab," they added.
Bahraini rights groups said authorities on Monday freed Ibrahim Sharif, former head of the secular Waad party, after he served a year in jail for what state news agency BNA said was publicizing "hatred of the regime".
A court shut down a main opposition society and the interior ministry stripped the spiritual leader of the island's Shi'ites of his citizenship in June.
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