The Epsom MP will work with Hutchinson Ports, according to the MP’s register of financial interests.
The company operates Thamesport, Harwich and Felixstowe.
The watchdog says that he won’t be able to speak about their commercial activities, and will focus on their environmental strategy.
Mr Grayling has been criticised for poor decisions, including awarding a contract to ferry operators with no ships.
They were later cancelled, and he described them as an insurance policy.