Farah, using the event as a warm-up for the Olympics in London, ran 13:29.91 to finish clear of Germany’s Arne Gabius, with Polat Arikan of Turkey taking bronze despite an early fall.
The pace was slow and Farah was content to sit on a narrow lead until the bell, when he began to extend his stride and really kicked with 300m to go.
He won by almost two seconds to retain his title but will expect much sterner competition in the Olympics.
In the men's 100m semi-finals, Harry Aikines-Aryeetey qualified for the final in a time of 10.30 when finishing behind favourite Christophe Lemaitre, but Mark Lewis-Francis was eliminated after finishing sixth in his semi-final with a time of 10.36.
Great Britain’s Luke Gunn suffered crushing disappointment in the steeplechase semi-final as he crashed into a barrier and missed out on the Olympics as a result.
But the big controversy of the day came as seven athletes were disqualified in the men's 400 metres heats for running outside their lanes.
Athletes were disqualified in all five heats of the 400 metres, with Brtain's Luke Lennon-Ford saying that it had been virtually impossible to stay in the lanes as the track was too narrow.
"One of the Finnish radio reporters told me they've changed the curve and they have blatantly made it too tight," he said.
"I was only going 85 percent around the top bend and had to cut back to 65 percent around the next."
His team mate Richard Buck was reinstated after successfully appealing the decision to disqualify him.