Al-Sadhan had been detained for 1,120 days with hearings held in secret prior to his conviction on Monday. His sister accused Saudi authorities of torturing the activist during his detention.
The sentencing was condemned by MENA Rights Groups, a collective of human rights lawyers working in the MENA region.
"We are appalled to learn that #AbdulrahmanAlSadhan was sentenced to 20 years in prison & a 20-year travel ban after a sham trial before the Saudi Specialised Criminal Court. He has been detained arbitrarily for over 3 years and deprived of his most basic rights," the group wrote on Twitter.
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Al-Sadhan's conviction is thought to be related to an anonymous Twitter account he allegedly operated, which commented on issues relating to human rights and social justice in Saudi Arabia.
The discovery of the account is believed to have occurred after a security breach at Twitter in 2015.
In August last year, Bloomberg News reported that data taken by two Saudi Twitter employees, who were working as spies, was handed to people with close ties to the Saudi royal family. This data directly led to the arrest of activists and critics who were allegedly using anonymous accounts on Twitter.
Areej Al-Sadhan, a US citizen, has been a vocal supporter of her brother's case.
"THIS IS INSANE!!! They are seeking 20 YEARS imprisonment followed by 20 YEARS of travel ban!!!!!" she wrote following the news of his sentencing.
In March, she condemned the hearing that led to his conviction.
"A secret hearing took place yesterday without informing my father! The lawyer was informed last minute & when he arrived the hearing was over!!" she wrote on Twitter.
Read more: How Saudi Arabia's fear of feminism landed Loujain al-Hathloul in a terror court
Following the news of the sentencing, Lina Al-Hathloul, sister of women's rights activist Loujain Al-Hathloul, voiced her support online and highlighted the continued repression of critics of the Saudi government.
The New Arab approached the Saudi embassy in the UK for comment on the case but did not receive a response at the time of publication.
The US State Department issued a condemnation of the sentencing of Al-Sadhan.
"We are concerned by reports that a Saudi counterterrorism court sentenced Saudi aid worker Abdulrahman al-Sadhan to 20 years in prison followed by a 20-year travel ban. We will continue to monitor this case closely throughout any appeals process," said Ned Price, State Department spokesperson in a statement.
"As we have said to Saudi officials at all levels, freedom of expression should never be a punishable offense. We will continue to elevate the role of human rights in our relationship with Saudi Arabia and encourage legal reforms that respect the human rights of all individuals."
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