Mehbooba Mufti, former J&K chief minister, condemned the journalist's arrest.
Fahad Shah, editor of news portal, The Kashmir Walla, has been arrested for sharing "anti-national" content on social media, a police spokesman said.
In a statement, police said the social media posts by Fahad Shah "tantamount to glorify terrorist activities and intending to create fear among the public to disturb law and order".
"During the course of the investigation, one accused, identified as Fahad Shah, was arrested. The accused is on police remand. The investigation into the matter is under way," said police.
Mehbooba Mufti, former J&K chief minister, condemned the journalist's arrest.
"Standing up for the truth is deemed anti national. Showing the mirror to a deeply intolerant & authoritarian government is also anti national. Fahad's journalistic work speaks for itself & depicts the ground reality unpalatable to GOI. How many Fahad's will you arrest?" tweeted Ms Mufti.
Police, however, has said that the news portal has been uploading content with criminal intent.
"Some Facebook users and portals have been uploading anti-national content including photographs, videos and posts with criminal intention to create fear among the public and the content so uploaded can provoke the public to disturb law and order," said the police.
"Uploading such posts tantamount to glorifying the terrorist activities and causing dent to the image of law enforcing agencies besides causing ill-will and disaffection against the country," the statement said.