Transport for West Midlands said buses were not stopping at the airport or the international bus station.
One passenger told ITV News Central they have been evacuated to a hall at the NEC and there is a bomb squad on site at the airport.
A spokesman for West Midlands Fire Service said: “Shortly after 12.20pm today, we were notified of an ongoing incident at Birmingham Airport.
“Two officers are currently on site, working closely with police, ambulance, and airport staff. Local resources are on standby, ready to provide additional support if needed.”
Last month the arrivals zone was evacuated at the airport while police carried out an investigation.
Flight were also suspended after a security incident in April, which turned out to be a false alarm over a suspicious item on a flight to Belfast.
Birmingham Airport said later on Wednesday that flights would “slowly be returning to normal” after police completed their investigation.
A Birmingham Airport spokesperson said: “Following a police investigation, some passengers were evacuated and we had to suspend departing flights for a couple of hours.
“That investigation has now concluded, and operations will slowly be returning to normal. All passengers are advised to check the latest flight information from their airline.
“Whilst we apologise for any inconvenience and disruption, the safety and security of everyone at the airport was our number one priority as we worked through this incident with police partners.”