1117 GMT January 19, 2019
"I am leaving the post of the country's leader," Sarkisian was quoted as saying in a statement by his press service, just days after he took office, AFP reported.
"Nikol Pashinyan was right," he said referring to the leader of the protests. "I was wrong."
After serving for a decade as president, Sarkisian was last week elected prime minister by lawmakers in a move the opposition said was designed to extend his rule under a new parliamentary system of government.
Constitutional amendments approved in 2015 transferred power from the presidency to the premiership.
Sarkisian, a shrewd former military officer, was first elected president of the impoverished country in 2008.
After the 2008 vote, 10 people died in clashes between police and supporters of the defeated opposition candidate.
He was reelected in 2013, with his second and final term ending April 9.
Earlier Monday, protest leader Pashinyan was freed after police detained him Sunday following failed talks with Sarkisian.
"So has everyone now understood that we have won?" Pashinyan told supporters shortly after his release in the capital Yerevan and before Sarkisian resigned.
Flag-waving supporters cheered the 42-year-old opposition MP and some kissed and hugged him on the 11th day of demonstrations in the country of 2.9 million people.
Prosecutors had said that Pashinyan and two other opposition politicians "were detained as they were committing socially dangerous acts".
Protesters had denounced Sarkisian and called on the 63-year-old to stand down.
Sarkisian had earlier refused to go and on Sunday stormed out of televised talks with Pashinyan, accusing him of "blackmail".
Earlier Monday, groups of young men briefly blocked roads in Yerevan and hundreds of students marched arm-in-arm holding Armenian flags.
A group of serving soldiers joined the protests while the country's Defense Ministry threatened to take "serious measures" against them.
A number of uniformed former soldiers and veterans who fought in Nagorny Karabakh – a breakaway region seized by Armenian separatists at the end of the Soviet era – also marched with the protesters.
Tens of thousands had rallied in Yerevan over the weekend against pro-Kremlin Sarkisian.
Pashinyan, the leader of the Civil Contract Party, last week called for the "start of a peaceful velvet revolution."
Hundreds of people were detained at protest rallies across Yerevan on Sunday.
Earlier Monday, Armenia's Investigative Committee said 26 had been detained on suspicion of "hooliganism" and use of violence against police.