The Treasury Department said Exxon had business dealings with Igor Sechin, president of the Russian state energy company Rosneft and a former adviser to Russian President Vladimir Putin, after he was blacklisted under sanctions imposed in the wake of the Russian annexation of Crimea in 2014.
Treasury's Office of Foreign Assets Control, the principal agency overseeing US sanctionsprograms, said Exxon's actions had caused "significant harm" to the Ukraine sanctions program and the company's "senior-most executives" were aware of the activity.
The violations "constitute an egregious case" and the company showed "reckless disregard" for US rules, the office said in a statement.
Two US subsidiaries of Exxon signed eight legal documents with Sechin in May 2014 for oil and gas projects in Russia, after he was included in the sanctions in late April, according to OFAC.