Icahn will be a special adviser to the president-elect overseeing regulation specifically, according to the transition.
The investor has long been a critic of various government regulatory agencies, most notably the Environmental Protection Agency.
"Carl was with me from the beginning, and with his being one of the world's great businessmen, that was something I truly appreciated," Trump said in a release from the transition.
"He is not only a brilliant negotiator, but also someone who is innately able to predict the future, especially having to do with finances and economies. His help on the strangling regulations that our country is faced with will be invaluable."
According to a report from The Wall Street Journal, Icahn has already assisted the transition with vetting candidates for the EPA chief. The nomination eventually went to Scott Pruitt, the attorney general of Oklahoma. Icahn's experience with the EPA comes from several investments in oil and gas companies.
Icahn will also have a role in selecting the new head of the Securities and Exchange Commission, according to the report.
Icahn is a longtime friend and supporter of Trump, and an advocate for the president-elect since before the Republican primaries. The hedge fund legend has said Trump will bring economic growth back to America by rolling back regulation and giving companies the ability to invest again.
"I am proud to serve President-elect Trump as a special adviser on regulatory reform," Icahn said in a press release. "Under President Obama, America's business owners have been crippled by over $1 trillion in new regulations and over 750 billion hours dealing with paperwork. It's time to break free of excessive regulation and let our entrepreneurs do what they do best: create jobs and support communities."
The role, according to the Trump announcement, will not be an official government position and will not have any "specific duties."
Icahn is estimated to be worth about $16.7 billion, according to Forbes. Icahn's appointment adds another billionaire to Trump's top advisory team, a group that already has a net worth of well over $10 billion.
Icahn's investment fund fell 12.7% this year through the end of September, according to a regulatory filing.