Hong Kong's chief executive, CY Leung, received millions from an Australian publicly-listed firm, according to Fairfax media.
Secret documents seen by the media group are said to reveal payments from engineering company UGL to Mr Leung of £4m (US$6.4m).
His office said in a statement the payments were from his resignation from the company not for future services.
It comes as Mr Leung, who is backed by Beijing, faces pro-democracy protests.
The money was paid to him in 2012 and 2013 after he became the leader of Hong Kong, according to Fairfax. The payment was not declared on his register of personal interests.
UGL acquired a property services firm, DTZ Holdings, where Mr Leung was Asia Pacific director.
This agreement was made separately between UGL and Mr Leung to ensure he did not compete with UGL and to act as "a referee and advisor from time to time", according to documents seen by Fairfax media.
However, Mr Leung's office said that he did not provide any services after his resignation which took effect on 4 December 2011.
Analysis: Juliana Liu, BBC News, Hong Kong
Besides struggling to contain a nearly two-week long civil disobedience movement, CY Leung is now dealing with revelations that he accepted secret payments from a listed conglomerate.
The news has been widely picked up by Hong Kong press and quickly shared via social media networks. At least one of his critics has called it a political scandal.
Having been elected in 2012 with just over half of the votes from a committee of 1,200 members who were largely loyal to the Chinese government, Mr Leung already lacks legitimacy in the minds of a significant portion of the public.
That he was not obliged to declare any of the payments under Hong Kong's current system will not excuse his behaviour in their eyes.
"According to an agreement between UGL (which was at that time acquiring DTZ) and Mr Leung on the conclusion of his employment with DTZ, UGL undertook to make payments to him over two years and to underwrite for DTZ the payment of outstanding agreed bonus," said Mr Leung's spokesperson Michael Yu in a statement.
"The payments therefore arise from Mr Leung's resignation from DTZ, not any future service to be provided by him.
"Both the resignation from DTZ and conclusion of the agreement with UGL took place before Mr Leung was elected as the chief executive. There is no requirement under our current systems of declaration for Mr Leung to declare the above."
Speaking to Fairfax media, UGL said no clause had been put in place to make the payment invalid if Mr Leung was elected to public office, because managers thought he would not win.