The navigation system of a plane carrying Ursula von der Leyen was disrupted due to suspected Russian interference, the European Commission said.
A spokesperson said the "GPS jamming" happened while the Commission president was about to arrive in Bulgaria on Sunday, but she still landed safely.
They added: "We have received information from the Bulgarian authorities that they suspect that this was due to blatant interference by Russia."
The Financial Times, citing unnamed officials, reported that von der Leyen had to land at Plovdiv Airport using paper maps.
The European Commission said "threats and intimidation are a regular component of Russia's hostile actions" and that the incident would reinforce its commitment to "ramp up our defence capabilities and support for Ukraine".
