Radev has indicated he may contest an upcoming election that's widely expected after protests forced a centre-right government to step down. The president was an opponent of the government and threw his support to the protests in early December.
In a televised address, Radev said that he will formally submit his resignation to the Constitutional Court on Tuesday.
Under the constitution, the current vice president, Iliana Yotova, must be sworn in by parliament to take the post until the end of the presidential mandate.
"The battle for the future of our homeland lies ahead, and I believe we will face it together with all of you - the worthy, the inspired, and the unyielding! We are ready. We can, and we will succeed!" Radev said in his address.
Radev's decision comes amid public expectation that he will form a new political party.
His resignation, the first by a head of state in Bulgaria's post-communist history, comes as the country - which is a member of the European Union and NATO - struggles to overcome a prolonged political crisis.
Large anti-corruption protests last month forced the resignation of the governing coalition, led by the centre-right GERB party. Attempts to form a new government within the current parliament have subsequently failed, and the country is headed towards its eighth parliamentary election since 2021.
Radev, whose second mandate ends in 2026, has repeatedly indicated he may take part in new elections.
The 62-year-old former Air Force general, has been a vocal opponent of the leader of the GERB party, Boyko Borissov, and of politician and oligarch Delyan Peevski, who has been under US and UK sanctions, and whose MRF New Beginning party has repeatedly backed the outgoing GERB-led coalition.
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