Rabbah al-Quwaii, a journalist at al-Shams newspaper in Riyadh, was arrested by Saudi Arabia's domestic intelligence agency, al-Mabahith, on April 3 and also charged with "doubting the [Islamic] creed", the group said.
Saudi government officials confirmed the arrest and said al-Quwaii faced prosecution in court on unspecified anti-Islamic charges.
Al-Quwaii was the editor of the media section of the Saudi internet forum, A Body of Culture.
His writings questioned whether official religious doctrines in the conservative Gulf state had contributed to acts of violence carried out by al-Qaida in the Arabian peninsula, Human Rights Watch said.
Al-Quwaii had also received death-threats which he reported to al-Mabahith in November last year, but said they took no action.
However, the governor of the Saudi province of Hail did order an investigation after al-Watan newspaper reported that al-Quwaii's car had been damaged and a note left saying "next time it's you", the group said.
"Saudi security forces apparently believe they are there to abuse citizens like Rabbah al-Quwaii rather than protect them," said Sarah Leah Whitson, a Human Rights Watch director.
The group also called on Turki al-Sudairy, Saudi Arabia's minister for human rights, to open a public inquiry in the role of the intelligence services in al-Quwaii's arrest.