The state security court's ruling against Mansoor, who prior to his imprisonment had been one of a tiny number of publicly active rights campaigners in the UAE, was confirmed by the state news agency WAM.
"We are concerned that Mansoor’s conviction and harsh sentencing relate to his exercise of the right to freedom of expression and opinion," U.N. human rights spokeswoman Ravina Shamdasani told reporters in Geneva.
"We urge the government of the UAE to promptly and unconditionally release Mansoor and to ensure that individuals are not penalized for expressing views critical of the government or its allies," she said.
The ruling, upholding the May 2018 conviction, which is not subject to appeal, was accompanied by a fine of 1 million dirham ($270,000).
A trade and tourism hub, the UAE is an absolute monarchy which tolerates little public criticism.
"This is certainly not a one-off case in the UAE," Shamdasani said.
Prominent critic Nasser bin Ghaith was sentenced in March 2017 to 10 years in prison for tweeting about human rights violations in Egypt and criticizing the politicization of the judiciary in the UAE, she said.
Mansoor, a 49-year-old electrical engineer and poet, was among five activists convicted of insulting the UAE's rulers in 2011. They were pardoned the same year.
He was arrested again in March 2017 on charges of publishing false information and rumors, promoting a sectarian and hate-incited agenda, and using social media to "harm national unity and social harmony and damage the country's reputation."In 2015 Mansoor received the Martin Ennals Award for Human Rights Defenders, an annual prize awarded in Geneva by a panel of international human rights organizations, for his work documenting the human rights situation in the UAE.