East 17 singer Terry Coldwell, 43, saw the 27-year-old man holding the blade and shouting aggressively in the departure hall at Dusseldorf Airport.
The suspect was quickly cornered by 10 armed police officers around 9.15am today.
They eventually convinced him to put down the weapon and arrested him.
Terry, from east London, said: "We could see something was happening because a few police officers were crowding around this man."
SWNSSCARY: A man was arrested with a knife at Dusseldorf Airport
He added: "He was shouting, I'm not sure what he was saying because it was in a different language, but he sounded angry.
"The police were trying to calm him down and gesturing for him to put the knife on the floor.
"Eventually he did. It was like he thought 'this isn't worth it' and changed his mind. They quickly pounced on him and arrested him.
"It was terrifying. With all the recent terrorist attacks, we couldn't help but think the worst."
“It was terrifying”Terry Coldwell
Terry was arriving at the airport to return to London Heathrow when the man brandished a knife.
The suspect was alone and didn't appear to have any baggage with him.
A spokesman for Dusseldorf Airport confirmed the incident was not terror related.
GETTYSCARED: East 17's Terry Coldwell caught the incident on camera
He added: "I can confirm that a 27-year-old man in possession of a knife has been arrested in the public area of our airport this morning.
"The man was not threatening other people but there was a high probability the he wanted to harm himself with the knife.
"Therefore policemen arrested him and send him into psychiatric hospital."
This comes as a passenger attempted to open a London-bound plane's door in mid-air.