The tusker was released from its enclosure to graze on September 9 but it did not return, reserve field director Vivek Jain told PTI.
The pachyderm was found dead floating in river Ken in Mandla range of the park yesterday, he added.
He said PTR assistant director Pratibha Shukla has been asked to inquire into the cause of the elephant's death.
Besides, the viscera of the animal has been sent to laboratories in Jabalpur and Sagar districts to ascertain the actual cause of the death, Jain added.
The tuskers are used for patrolling and tiger protection.