Figures from the Hong Kong Airport Authority showed 3.6 million passengers came through the city's airport during the month compared with 3.8 million a year earlier.
Cargo also dropped, with 317,000 tonnes handled in September, down 7.5 per cent on the previous year, the authority's figures showed.
The number of flights edged down 0.4 per cent to 24,570.
The reduction in passengers was mainly the result of fewer visitors to the territory, while travel by Hong Kong residents and transit passengers also decreased slightly, the figures showed.
On the cargo front, the decline was widespread, including import, export and trans-shipment.
'The steep decline in traffic volumes in both passenger and air cargo clearly reflected a very troubled global economy as a result of the financial tsunami, which has affected all countries in the world,' Mr Stanley Hui, chief executive officer of the airport authority said in a statement released on Sunday.
'The US and Europe, the biggest trading partners with Hong Kong and China, saw a significant drop in air cargo traffic to/from Hong Kong in the month.'
Mr Hui said the annual 'Golden Week' holiday also saw fewer visitors arrive by air than in 2007, showing a continuing decline in traffic between Hong Kong and mainland China.
'Given the very difficult operating environment and shrinking demand in the economic slowdown, airlines are expected to further scale back their operation or put on hold their expansion plans,' Mr Hui said.
'The aviation industry is expected to face even more difficult times in the coming months.'
Hong Kong's carrier Cathay Pacific said last week its passenger numbers in September had dropped 0.7 per cent year-on-year, with business and first class travel hit particularly hard. -- AFP