His criticism adds to a growing tide of accusations that the United States and its key ally Britain deliberately manipulated information to make it look as if Iraq had weapons of mass destruction.
The fear that Iraqi leader Saddam Hussein had such arms at his disposal was one of the chief justifications for the war to topple him.
The German inspector, Peter Franck, was part of the UN weapons inspection team in Iraq from December last year until shortly before the US-led invasion in March.
He told Der Spiegel that US Secretary of State Colin Powell did not present truthful evidence to the UN Security Council in a famous February 5 speech.
It was "all a big bluff," Franck said.
"Basically, it was all a show for the American public.
He said Powell used satellite pictures to try to show that decontamination trucks in front of an ammunition bunker were proof that Iraq was experimenting with chemical weapons there.
However, an earlier visit by UN inspectors had already determined that the trucks were firefighting vehicles.
"What Powell said simply wasn't true," Franck told the magazine.
He said US officials exaggurated the numbers of soldiers and equipment Iraq had at its disposal. A UN inspection of an air defence base showed the United States had over-estimated the number of planes there by five times.
Franck said US officials appear to have concentrated too much on satellite images, which could be interpreted different ways.
London and Washington have strongly denied claims that they manipulated any evidence.
US Defence Secretary Donald Rumsfeld insisted the intelligence was "good," and promised Thursday that Powell's testimony "will be proved right."
The administration of US President George W. Bush has been under increasing pressure to explain why none of the weapons Iraq was alleged to have possessed have been found in the six weeks since the end of the war, despite intense searches of suspect sites.