The record cold spell has seen winter jackets sales more than double from last year, while demand for heaters has been so great that many appliance stores have nearly sold out.
Columbia Sportswear and Pro Cam-fis said the rush on their thick winter jacket range came just before the Lunar New Year.
The cold weather warning was issued on January 24 and the Observatory has predicted the coldest weather spell to hit the city in 40 years will continue into next week.
A spokeswoman for Pro Cam-fis said the most popular items were down and fleece jackets. Sales of these and other jackets had more than doubled.
Sales reached their peak early this month and were now declining as most expected the cold weather to end soon.
Columbia Sportswear Company said sales had tripled since last year. Company general manager for Hong Kong and China Alice Lai Wai-hing said the most popular jackets were from the higher-priced Titanium Series, which offered multiple layers to cope with different weather conditions.