Guotai Junan said it had been unable to get in contact with Yin Fung, chairman of the board, executive director and chief executive since Wednesday 18 November, in a statement to the Hong Kong Stock exchange today.
However, it assured investors that operations of the company remained normal and stable.
"The news had a significant impact on Guotai Junan stock as the whole event remained unclear. The chairman has been missing for almost a week, which would arouse investors' concerns," Patrick Yu, associated director of Cash Asset Management in Hong Kong, said.
Guotai Junan has charged its deputy chief executive, Wong Tung Ching, with leading the company in Fung's absence.
"As Dr Yim currently cannot discharge his duties, the board resolved on 22 November 2015 to appoint Ms Qi Haiying, executive director and deputy chief executive officer of the company, to temporarily act as chairman of the board until further notice," it said.
"In addition, the board resolved to appoint Mr Wong Tung Ching, executive director and deputy chief executive officer of the company, to temporarily act as chief executive officer of the company until further notice."
The Hong Kong unit is majority owned by Guotai Junan Securities, one of the biggest security firms in China, which listed on the Shanghai stock exchange in June in China's largest initial public offering since 2010.