Mr Viggers will also step down from the trust’s council of governors at the end of May.
He has been chairman of the Western Sussex, which runs St Richard’s, Worthing and Southlands hospitals, since 2011, a period in which the trust became only the third in England to be named ‘Outstanding’ by the Care Quality Commission in 2016.
Mr Viggers is also retiring from the same position at Brighton and Sussex University Hospitals NHS Trust (BSUH).
He said: “The decision to retire has been the most difficult of my working life.
“I have spent seven wonderful years with such outstanding people at both Western Sussex and BSUH.
“Witnessing the dedication staff in our hospitals show to their patients day in and day out has been a truly humbling experience and I thank them all for that incredible commitment.
“However, at the age of 65, I feel the time is right to spend more time with my family and grandchildren, and allow another the privilege of chairing these two great organisations and helping them continue improving care for patients.”
Trust chief executive Marianne Griffiths paid tribute to Mike and his own contribution to the two organisations, saying: “We often talk about our staff going the extra mile for patients, but that absolutely applies to Mike’s own commitment to them too.
“We couldn’t have wished for a better and more dedicated chairman than Mike, and we will all miss him as a colleague, a supporter and a friend.
He will be a hard act to follow.
We managed to persuade him to stay on beyond his original term once and I’m only sorry that we haven’t been able to do so again.”
She added: “We all wish Mike a very happy retirement and thank him again for his service to our hospitals, patients and community. We hope to appoint his successor very soon.”