Lee Man-ho wrote on social media on Thursday that he had decided against taking the loyalty oath, arguing that he was elected by the public in 2019 at a time when no such oath requirement was in force.
The government has said any councillor who fails to take the oath will be removed from office.
Members of the Yau Tsim Mong, Sham Shui Po, Kowloon City, Wong Tai Sin and Kwun Tong district councils have been told to take the oath at North Point Community Hall on Friday.
Last week, the government rejected the pledges made by seven Hong Kong Island district councillors and disqualified them from office. Another member was ousted after deciding not to take the oath.
More than half of the territory’s district councillors had already resigned before the oath-taking requirement came into effect, with several others disqualified after either leaving Hong Kong or ending up behind bars.