"Bulgaria will take charge of the EU presidency at a key moment for the union ... May the slogan 'strength in unity' guide us," conservative and pro-EU Prime Minister Boyko Borisov wrote in a post on Facebook.
"I am confident that we shall work with success on our priorities, on continuity," added Borisov.
The presidency, which rotates between different EU member states, will give Bulgaria the opportunity to chair meetings and set agendas, as the bloc grapples with the record influx of migrants, management of its borders, rising populism and Britain's EU divorce.
The system currently puts too much pressure on frontline states Greece and Italy, but countries have been divided over how to replace it.
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To solve the refugee crisis, Sofia is also expected to push for normalised ties with Turkey, with which Bulgaria shares a 260-kilometre border.
Relations have soured dramatically between the bloc and Turkey since the failed coup against president Recep Tayyip Erdogan in 2016.
A net recipient of EU funds, Bulgaria backs further cohesion between member states despite a likely reduced budget across the bloc by 2020 as a result of Brexit.
Sofia is also expected to use Brexit as an opportunity to push for closer EU ties with Western Balkan countries.
It hopes to reignite accession talks with Montenegro and Serbia, while helping Albania and Macedonia move closer to starting EU membership negotiations, and will host the first EU-Balkans summit in 15 years in May.
Sofia will mark the start of its presidential duties with a concert today at the capital's Palace of Culture featuring contributions from each member state.
Taking up the presidency also gives Bulgaria an opportunity to improve its image as the bloc's most corrupt country.
It wants to convince peers that after ten years in the EU, it is finally ready to join the visa-free Schengen travel area and move towards entering the eurozone.