A 70-year-old man, who had travelled to the Chinese cities of Shenzhen and Zhongshan earlier in December, tested positive for the H7N9 strain of the virus, the government said in a statement.
He said he had come across mobile stalls selling live poultry in Zhongshan, but authorities said they were still investigating the source of the infection.
The patient has been hospitalised in stable condition. Those who have been in close contact with him have been put under medical surveillance although none have yet reported any symptoms.
Cases of bird flu were "expected to increase in winter based on its seasonal pattern", the government statement added.
Another elderly man, who had bought a chicken from a market in Guangdong province, died on Sunday, less than a week after testing positive for H7N9.
Hong Kong is particularly alert to the spread of viruses.
Bird flu was first reported in humans in Hong Kong in 1997, when six people died, and subsequent outbreaks have killed hundreds more worldwide.
An outbreak of Severe Acute Respiratory Syndrome (SARS) swept through the city in 2003, killing 299 people.
Bird flu scares in the past two years have seen mass culls of up 20,000 birds in the city.
