Monsanto Co. and a Dow Chemical Co. unit filed the lawsuit in federal court in Honolulu on Thursday. They're asking a judge to immediately prevent the law from taking effect and to invalidate the measure.
County voters narrowly passed a ballot initiative last week that imposes the ban.
Monsanto Vice-President John Purcell says the law interferes and conflicts with long-established state and federal laws that support the safe and lawful cultivation of GMO plants.
The initiative's authors sued the county in state court Wednesday to ensure the county implements the law.
Maui County spokesman Rod Antone says the county is unable to comment because of pending litigation.