The charges included insulting the "Divine Self" and the Prophet Mohammed, mocking the Quran and spreading atheism.
His sister, Raeda Fayadh, appealed to King Salman bin Abdulaziz Al Saud of Saudi Arabia to grant clemency to her brother in an interview Monday with CNN.
She said her brother had done nothing wrong, having been falsely accused by a man with a personal animosity toward him.
There was nothing blasphemous in her his writings, she said, and those who had accused him were damaging the image of Saudi Arabia, she said.
"He wrote in words that stupid people misunderstood," she said.
Accusations of inappropriate relations
Fayadh had originally been handed a lesser sentence of four years in prison and 800 lashes on the charge of having inappropriate relations with members of the opposite sex.
The court had dismissed a prosecution request for a death sentence on the apostasy charge, citing Fayadh's declared "repentance" and testimony that indicated hostility between Fayadh and the complainant, Human Rights Watch said.
But the prosecutor appealed the ruling, according to the rights group. In mid-November a new judge dismissed the earlier decision, saying Fayadh's repentance was not enough to avoid a death sentence for apostasy, citing verses from the defendant's poetry.
An English translation of the poems
has been posted on the blog of U.S.-based activist and writer Mona Kareem.
Fayadh was initially arrested in 2013 after a personal disagreement with a man at a coffee shop in the town of Abha in relation to his poetry, according to Human Rights Watch, writers' association PEN International and the poet's sister.
Raeda Fayadh, who spoke to CNN from her home in Gaza, said the men had been watching a European soccer match at the coffee shop when the fight started.
The man reportedly complained to the ultraconservative kingdom's religious police, accusing Fayadh of insulting Islam in his book of poems and insulting the Saudi state.
Photos with women
Fayadh reportedly has 30 days to appeal the death sentence after which the case goes to the Supreme Court, Human Rights Watch reported.
After Fayadh was arrested, according to the rights group's account based on the court documents, the religious police discovered on his phone photos of Fayadh with several women, whom Fayadh said he met at an art gallery. These photographs were the basis for the charge of having inappropriate relations with members of the opposite sex.
Raeda Fayadh said the pictures were "all of his friends, artists that he worked with" and that there was nothing improper about them.
"If you open the net you will find the same pictures, it's not pictures of naked women or anything -- they are all normal pictures," she said. "There is no issue at all."
Activists have posted scans online purporting to be the original trial documents
. CNN cannot verify the authenticity of the documents, but they appear to match Human Rights Watch's reporting.
CNN has reached out to Saudi officials for comment on Fayadh's case but hasn't received a response.
Writers, rights groups 'appalled'
The sentence has drawn condemnation from rights groups, cultural figures and social media, with the hashtag #FreeAshraf
appearing in response to reports about it.
Prominent writers, including Britain's poet laureate Carol Ann Duffy, and Syrian poet Adonis signed an open letter by PEN International saying they were "appalled" by the judgment.
"It is not a crime to hold an idea, however unpopular, nor is it a crime to express an opinion peacefully," read the letter.
An online petition launched by Amnesty International
calling for his release has already reached more than 22,000 signatures.
"The sentencing of Ashraf Fayadh to death after an unfair trial shows the deep flaws in Saudi Arabian criminal justice system and is yet another appalling example of the authorities flouting their international human rights obligations," said Sevag Kechichian, Amnesty International's Saudi Arabia researcher.
"Fayadh was imprisoned solely for peacefully exercising his right to freedom of expression -- he should be immediately and unconditionally released and his conviction should be quashed."
Sarah Leah Whitson, Human Rights Watch's Middle East director, said that "regardless of what Fayadh said or didn't say, Saudi Arabia should stop arresting people for their personal beliefs."
"The fact that Ashraf Fayadh is facing the prospect of being beheaded only adds to the outrageousness of this court ruling," Whitson said.
Fayadh, a Saudi-born Palestinian, has been a prominent figure in Saudi Arabia's nascent contemporary art scene, where he was involved in the Saudi-British art group Edge of Arabia and curated its exhibit for the 2013 Venice Biennale
Saudi Arabia has faced increasing criticism for the punishments handed down by its judicial system, including beheadings, crucifixions and floggings. Raif Badawi, a liberal Saudi blogger
, was handed a 10-year prison term and ordered to be lashed 1,000 times last year for "insulting Islam."