HRW denounces detention of Saudi cleric over 'pro-Qatar' tweet
The Saudi authorities have also imposed arbitrary travel bans on 17 of his relatives without showing "the slightest evidence or accusation of wrongdoing on their part", said HRW.
The rights group said in a statement on Sunday that Salman al-Awdah is among dozens arrested since mid-September.
The rights group, speaking to a family member of Awdah, says he has not been questioned or charged since arrest four months ago and has been held in solitary confinement.
He has been allowed just one 13-minute call since his detention in October.
His brother, Khaled al-Awdah, remains imprisoned for tweeting about the cleric’s arrest, which occurred after he expressed support for Saudi reconciliation with Qatar.
The cleric, known for advocating reforms in recent years, had millions of followers on Twitter.
A family member told HRW that Awdah was being held over his refusal to comply with an order by Saudi authorities to tweet specific text supporting the Saudi-led blockade of Qatar.
Awdah posted a tweet on 9 September, saying: "May God harmonise between their hearts for the good of their people" - an apparent call for reconciliation between the Gulf countries, HRW said in a statement.
"Crown Prince Mohammad bin Salman's efforts to reform the Saudi economy and society are bound to fail if his justice system scorns the rule of law by ordering arbitrary arrests and punishments," said Sarah Leah Whitson, Middle East director at HRW.
"There's no justification for punishing family members of a detainee without showing even the slightest evidence or accusation of wrongdoing on their part."
"If Mohammed bin Salman wants to show that a new era has begun in Saudi Arabia, a refreshing first step would be the release of activists and dissidents who have never been charged with a recognisable crime and should never have gone to jail in the first place," Whitson said.
Saudi Arabia, the UAE, Bahrain and Egypt imposed a sea, air and land blockade against Qatar on 5 June, accusing Doha of aiding "terrorists" and having close ties with Iran. Qatar denies the allegations.