The party that holds the majority of seats in Armenia's parliament says it now considers itself the opposition and won't cooperate with the new government.
59 MPs voted in favor, 42 voted against.
On Tuesday morning, ahead of the vote, thousands of Pashinyan supporters gathered in Yerevan's central Republic Square, sporting white t-shirts with their leader's portrait or dancing traditional Armenian dances and chanting "Nikol prime minister".
"Prime ministers will come and go", he told a rally in Yerevan. Pashinyan was the only candidate, but members of the ruling Republican party did not vote for the opposition.
The first vote in Parliament took place on 1 may.
"All people are equal before the law, period!" he said. Elections will not be rigged and vote bribes will not be handed out anymore, period! "Human rights will be protected, government will not be a means for making money, and corruption will be rooted out, period!" "In Armenia, the page of political and economic persecution has been closed". Pashinian earlier said that he would push for recognition of the region's sovereignty. But he gave no possible dates for the conduct of such polls. "However, if it turns out that they are ineffective, they will be discontinued", he said.
The idea of fresh elections is also supported by businessman Gagik Tsarukian's alliance and the Armenian Revolutionary Federation.
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Mr Pashinyan has however shown a deft political touch over the past month, running a gauntlet of thorny political questions including whether Armenia should maintain close ties with Russian Federation and whether there should be a purge of former ruling party officials and a criminal trial into the deaths of protesters in 2008.
Under a 2015 referendum marred by irregularities, Armenia shifted powers from the presidency to parliament.
Pashinian declined to shed light on the composition of his cabinet when he spoke to reporters moments after being elected prime minister.
"On the other hand, as the protest movement has shown, he can be flexible", he said.
Supporters of opposition lawmaker Nikol Pashinian react in Republic Square in Yerevan, Armenia, Tuesday, May 8, 2018.
"The victory is not my being elected prime minister", he added. "It is coming from the Armenian streets, from the ordinary citizens".
It has been an unlikely rise to power in the post-Soviet republic of about three million for the former newspaper editor.