Passenger demand dropped 3.5 per cent last year, with companies struggling to fill more than three-quarters of seats on average, the IATA said.
Freight demand fared even worse, falling 10.1 per cent year-on-year and filling less than half available capacity.
IATA chief executive Giovanni Bisignani said: "In terms of demand, 2009 goes into the history books as the worst year the industry has ever seen.
"We have permanently lost two and a half years of growth in passenger markets and three and a half years of growth in the freight business."
Separately, aircraft maker Boeing gave a cautious outlook for the year ahead after dealing with production delays and declining orders in 2009. The group is expecting lower orders for its 777 aircraft, cuts in military procurement, and "market risks".
But the company managed to post a fourth-quarter profit of $1.27 billion (803 million), reversing an $86m loss from a year ago and beating analysts' expectations. Quarterly revenue rose 42 per cent to $17.9bn.