Three men were arrested after the Easyjet flight from Ljubljana to Stansted, in Essex, had been diverted to Germany's Cologne-Bonn airport on Saturday afternoon.
All 151 passengers were evacuated and flights were suspended for three hours.
A backpack belonging to the men was blown up by police.
A spokesman for Cologne-Bonn airport confirmed the landing, adding: "Prior to this, the pilot had been informed about a suspicious conversation on board, after which he decided to make an unscheduled landing in Cologne-Bonn.
"After the safe landing... the 151 passengers left the [aircraft] via emergency slides and were taken to a transit gate."
The passengers had to "undergo a check immediately" and additional security checks were carried out on the plane.
A statement from Cologne police said other passengers had made the crew aware of a conversation by three men.
It said the subject had been "terrorist content", without elaborating further.
It added that federal police had "blown up in a controlled manner" a backpack belonging to the men.
Easyjet said the captain had taken the decision to land as a precaution to allow additional security checks to take place.
All the passengers had been given a hotel for the night and would be flown back on Sunday morning, she said.
"We thank passengers for their understanding. The safety of Easyjet's passengers and crew is our highest priority."
A plane flying from Slovenia to the UK made an unplanned landing after the pilot was alerted to a "suspicious conversation" with "terrorist content".