The 41-year old Brnabic and her cabinet were backed by 157 MPs in the 250-seat parliament after being proposed by President Aleksandar Vucic.
Vucic had been serving as prime minister himself since 2014 until last month when he became head of state after winning the presidential vote in April.
Brnabic becomes Serbia's first female prime minister and the first openly gay premier in the Balkans, where homophobia remains widespread.
Serbia is home to about seven million people, most of them Orthodox Christians.
Brnabic's appointment is seen as Vucic's bid to please the EU, which Serbia hopes to join, amid concerns over the country's increasingly closer ties with Russia.
However Brnabic's cabinet remains very similar to Vucic's government, with a few new openly pro-Russian ministers, including defence minister Aleksandar Vulin and minister without portfolio Nenad Popovic.
The new premier entered politics last year as public administration minister following a career in the business sector and local development programmes.
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