The accused Abu Luhum is held in a jail in Najran, close to the border with Yemen. (Representational)
Human Rights Watch said Monday a Saudi court had sentenced a Yemeni man to 15 years for apostasy, urging the kingdom to prioritise decriminalising blasphemy amid a modernisation drive.
The rights group said that Ali Abu Luhum, 38, had been accused of making comments "via two anonymous Twitter accounts", which prosecutors argued were registered with phone numbers linked to him.
"The court found that the tweets were promoting 'apostasy, unbelief, and atheism'", HRW said, reporting the trial was held without defence witnesses.
Charges against Abu Luhum included the reported "denial of the existence of God" as well as publishing content which "prejudices public order, religious values, and public morals on social media", HRW said.
It said Abu Luhum was found guilty of "promoting atheism" and was sentenced to "15 years in prison for apostasy", but gave no further details of what he was accused of saying.
There was no immediate response from Riyadh.
The trial was held in October, with an appeal awaiting a decision by the Supreme Court, HRW said.
Abu Luhum is held in a jail in Najran, close to the border with Yemen.
The Sunni Kingdom of Saudi Arabia is leading a military coalition backing Yemen's internationally-recognised government in its battle against Iran-backed Shiite Huthi rebels.
Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman, has sought to project a moderate, business-friendly image of his austere kingdom as he seeks to boost investment to diversify the economy away from oil.
Saudi has also invested heavily in recent years in the tourism, entertainment and sports sectors.
"Saudi authorities are sparing no expense to portray the country as tolerant and reforming, but contradicting state orthodoxy on religion still results in a decade-and-a-half prison sentence," Michael Page, HRW's deputy Middle East, said in a statement.
"A 'modernising' Saudi Arabia needs to first stop policing people's personal beliefs," he added, calling for "the decriminalisation of blasphemy."