Video shared on social media purported to show the bullet-ridden body of Cherubin Okende, a former transport minister who was also a member of parliament, slumped inside his car. Reuters has not verified the footage.
Okende's Ensemble pour la Republique party said he was kidnapped from the parking lot of the Constitutional Court in the capital Kinshasa.
A source close to Okende said the politician had just dropped off a letter in response to a court summons.
Government spokesman Patrick Muyaya described the death as an assassination and said on Twitter it was a shock.
Okende resigned from the government at the end of December when Ensemble party leader Moise Katumbi left the ruling coalition and announced his intention to run against President Felix Tshisekedi.
Political tensions have risen in the run-up to the vote. Katumbi's adviser, Salomon Kalonda, was arrested at the end of May and charged with undermining state security over an opposition march that turned violent.
Kalonda, who was accused of bringing a firearm to the event, denies the charges.
The news of Okende's death sparked small anti-government protests in Kinshasa, with demonstrators burning tyres and shouting "assassin".
Katumbi called for independent investigations into what he described as an "odious crime".
Tshisekedi urged the judiciary to "shed light on this case" and sent his condolences to the family, the presidency said on Twitter.
The European Union ambassador to Congo and the United Nation's peacekeeping mission in the country also condemned the murder.
(Reporting by Stanislas Bujakera, Sonia Rolley and Ange Kasongo; Editing by Anait Miridzhanian, Sofia Christensen and Andrew Heavens)
