0742 GMT December 11, 2018
Lawmakers voted 59 to 42 to approve Pashinyan for the job, after the ruling Republican Party came around to backing his premiership bid on his second attempt, AFP reported.
The party had narrowly voted him down last week, plunging the Caucasus nation into its most serious political crisis in years.
"My first work after my election will be ensuring a normal life in the country," Pashinyan said ahead of the vote. "There will be no corruption in Armenia. Armenia will once and for all turn the page of political persecutions."
The 42 year-old added that Armenia's relations with Russia will "remain a priority".
"Military cooperation with Russia is an important factor in ensuring our country's security," Pashinyan said, referring to a two-decade long conflict his country is locked in with neighboring Azerbaijan.
"We will (also) be developing relations with European countries and the United States, with Iran and Georgia, China and India," he added.
Armenia is an ally of Russia, and Russian President Vladimir Putin congratulated Pashinyan in a statement released by the Kremlin on Tuesday.
The head of the ruling Republican Party's Parliamentary faction, Vagram Bagdasaryan, said his party backed Pashinyan to "ensure stability" in the country.
"We did not change our position. We are against Nikol Pashinyan's candidacy, but the most important thing for us is to ensure stability in the country," Bagdasaryan said ahead of the vote.
Pashinyan called for an end to the protests after the Republicans – who have 58 MPs in the 105-seat legislature – promised to back him in the second attempt.
The 42-year-old former newspaper editor also secured the support of two other major political parties – Prosperous Armenia and the Armenian Revolutionary Federation (Dashnaktsutyun) – which nominated him for the post together with his opposition Elk coalition.
Pashinyan was the only candidate for the premiership.
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".
Political analysts say Pashinyan's election is unlikely to put an end to the political crisis, as the ruling party retains a majority in Parliament and could well block his initiatives.
Analyst Vigen Akopyan said snap elections looked certain.
The hugely popular Pashinyan had in recent weeks piled pressure on the ruling party through an unprecedented campaign of civil disobedience, leading to the shock resignation of veteran leader Serzh Sarkisian, a week after he shifted to the newly-empowered role of prime minister after serving for 10 years as president.
Observers have expressed concern that the turmoil could destabilise the country locked in a decades-long territorial dispute with Azerbaijan.
Pashinyan's protest movement had accused Sarkisian of a blatant power grab.
In December 2015, controversial constitutional amendments initiated by Sarkisian were passed after a referendum that saw some 63 percent of the voters backing the country's transformation into a parliamentary republic with executive powers fully concentrated in the hands of a prime minister.