Salman al-Awda was among more than 20 people arrested in September in what the Saudi government has said was a crackdown on "intelligence activities... for the benefit of foreign parties against the security of the kingdom and its interests".
Awda's family, however, believes he is being held over a tweet linked to neighbouring Qatar, with which Saudi Arabia has cut all ties over accusations of links to Iran and Islamist extremists, according to HRW.
The cleric has reportedly been allowed one phone call, in October.
Saudi activists have said Salman al-Awda's brother Khaled has also been detained for disclosing that the cleric had been arrested.
Dozens of Saudi citizens have been convicted on charges linked to dissent and under the country's sweeping cybercrime law, particularly linked to posts on Twitter.
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