Left-wing website sendika.org said its editor-in-chief Ali Ergin Demirhan was held in a pre-down raid on its offices. It added that his hard disk and personal cell phone were confiscated in the raid.
Sendika.org said Demirhan was detained on accusations of "organising protests on social media while attempting to depict the results of the referendum as illegitimate" and "inciting people to enmity".
There have been daily street protests in several cities.
Turkish voters went to polling stations on Sunday to either support or reject government-backed constitutional changes that would convert the country's governance system into a presidential republic and therefore increase presidential powers. The fraudulent referendum results showed that the amendments to the constitution were approved by 51.41 percent of votes with all ballots counted.
Serious irregularities affected more than two million votes.