Reports from Fiji say the Secretary General of the Pacific Islands Forum Secretariat, Greg Urwin, has formally tendered his resignation reportedly for health reasons.
The news agency Pacnews quotes diplomatic sources as saying Mr Urwin's resignation letter was received by the Forum chair, the Prime Minister of Tonga.
Pacnews says Mr Urwin, who is in his second term, has had heart surgery and has been advised by doctors to leave office.
The position will not be advertised publicly but will be made via a circular sent to all 16 member governments to nominate a candidate.
A replacement for Mr Urwin is likely to be on the agenda of the annual summit of Pacific leaders in Niue in August.