Police spokesman Makhmadullo Asaduloyev said 50-year-old Urunboi Usmonov is suspected of being a member of Islamic movement Hizb-ut-Tahrir, which is illegal in the Central Asian republic.
According to police, Usmonov, a Tajik citizen, joined the banned group in 2009 to carry out "propaganda and promoting this movement in social networks." Calls to BBC seeking comment were not immediately returned.
Despite its designation as an extremist group, Hizb-ut-Tahrir has publicly foresworn the use of violence. It is an international pan-Islamic organisation that operates legally in many countries, including Britain.
Tajikistan holds strategic importance in the NATO military campaign in Afghanistan, serving as a link in supply routes.
As the impoverished former Soviet republic fights a strong Islamic insurgency at the Afghan border, authorities led by long-time President Emomali Rakhmon have maintained a tight grip over the media and civil rights groups.
Usmonov has been kept in a high-security prison in Tajikistan's north for two days, although the court only sanctioned his arrest today afternoon, the journalist's lawyer, Fayzinisso Vakhidova, told the Associated Press.
Vakhidova said Usmonov had been denied access to legal help for two days, adding that he is accused of using his job to promote the organisation's ideas.
