Mohammad al-Jishi, Rajab’s lawyer, said on Monday that the country’s criminal court decided to extend the detention pending further investigation.
The detention had earlier been extended until May 14.
The Bahrain Center for Human Rights, headed by Rajab, also confirmed the news.
Rajab’s family says he was arrested on April 2 for posting comments on Twitter denouncing torture in a regime detention center where activists are held.
Rajab was sentenced to six months in prison in January for posting tweets deemed critical of the Al Khalifa regime. Known internationally for his peaceful human rights work, Rajab spent two years in prison from mid-2012 to mid-2014.
He has been regularly targeted by the Bahraini government since pro-democracy protests broke out against the ruling Al Khalifa dynasty in the tiny Persian Gulf kingdom in early 2011, Press TV reported.
Scores of Bahrainis have been killed and hundreds of others injured and arrested in the ongoing crackdown on peaceful demonstrations.