Geisinger is seeking a court order to block Atlantic City, N.J.-based AtlantiCare from breaking away from the Danville, Pa.-based system, according to The Philadelphia Inquirer.
AtlantiCare's board of directors voted in September to separate from Geisinger, which acquired the New Jersey system in 2015.
Geisinger filed a lawsuit last week accusing AtlantiCare of violating the signed merger agreement by voting to disassociate from Geisinger.
The contract, signed in 2014, allows AtlantiCare to terminate the merger within 10 years, but only if Geisinger became controlled by a for-profit organization or affiliated with a religious organization. Neither of those circumstances occurred, according to the lawsuit.
The lawsuit also names AtlantiCare's chairman, Michael Charlton, and CEO Lori Herndon as defendants.
The claim doesn't say why AtlantiCare wants to regain its independence.
"The complaint filed in federal court on Thursday is related to discussions about the future status of the relationship between Geisinger and AtlantiCare. We continue to engage in discussions and are hopeful that we can resolve this matter to the satisfaction of all involved," a Geisinger spokesperson told the Inquirer.