The Storm issued a statement 24 hours out from their NRL clash with the Dolphins in Brisbane after the 66-year-old Bellamy underwent a series of recent tests.
"Over recent weeks, in consultation with specialists, Craig has undergone a series of medical tests and has since been diagnosed with a form of neurodegenerative disorder," the club said in a statement late on Thursday.
"He is receiving the best possible medical treatment and has been advised by specialists that his diagnosis will not have an impact on his ability to coach the team in the immediate future."
The club didn't provide any further details of the diagnosis but neurodegenerative disorders include Alzheimer's, Parkinson's disease, multiple sclerosis and Motor Neurone Disease (MND).
Melbourne have had an unsettling run of health and injury issues since their grand final loss to Brisbane in 2025.
As well as Bellamy, Eli Katoa underwent surgery to stop a brain bleed last November after a series of head knocks during an international, while fellow forward Tui Kamikamica is currently sidelined after suffering a stroke last month.
He also underwent brain and heart surgery.
In February, Bellamy signed a new contract through until the end of the 2028 season, with the grandfather of four working alongside his assistant coach son Aaron at the club.
He has coached 614 NRL games, only behind Wayne Bennett and the retired Tim Sheens, and boasts a win rate of almost 70 per cent, while he also played 150 matches for Canberra.
But the Storm are enduring a horror season, dropping six successive matches for the first time since Bellamy took over as coach in 2003.
A seventh-straight loss in Brisbane will equal the all-time club record set in 2002, with the usual title heavyweights sitting second-last on the NRL ladder, having won only two games.
Despite the recent woes, Melbourne chairman Matt Tripp said he believed Bellamy was still up to coaching at NRL level.
"Despite our recent results, I firmly believe Craig is still coaching at an elite level and I have no doubt he is the right person to drive the club forward," Tripp said in the statement.
"Craig has the full support of the board, players, coaches and staff to continue leading the club as he has done for the last 24 seasons."
Known for his fitness and strong work ethic Bellamy was at training on Thursday and showed no signs of ill health.
Melbourne skipper Harry Grant defended his coach following their loss last round against South Sydney when Bellamy was asked if he still had the "buy in" of his team.
"Yeah he does, that's a silly question. You don't need to ask him that," Grant said.
"He's a modest guy and he's probably going to look to himself for these results, but at the end of the day no one works harder than Craig.
"You come into the club every morning and he's in here working."
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