Ironman Wales is the latest casualty of the coronavirus pandemic.
Organisers of the Tenby event, that attracts thousands of entrants from across the country every year, say it will not be taking place on September 6 as planned.
It has not been postponed, but they have said they will return on September 12, 2021.
A statement said: "With the health and safety of our community being an utmost priority, and in alignment with Pembrokeshire County Council in relation to the COVID-19 pandemic, we can confirm that the Ironaman Wales triathlon originally scheduled for September 6 cannot take place in 2020.
"In what has been a continually evolving and challenging time globally, we recognise that this decision due to the above-mentioned unforeseen events may come as a disappointment.
"While we are prevented from holding the event on the original date, we are looking forward to providing athletes with an exceptional race experience at IRONMAN Wales in 2021."
Look how the Ironman competition takes over Tenby every year:
Athletes who have entered should have received an email with further details.
Anyone who needs additional information is asked to contact wales@ironman.com.
The news comes a week after it was announced that this year's Cardiff Half Marathon is being postponed until 2021 because of the coronavirus pandemic.
Organiser Run 4 Wales said holding it in just four months' time "really isn't feasible" and the event and a new date has been set.
It will now take place on March 28, 2021.
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The Newport Wales Marathon - already postponed from April to October - has also been pushed back again to a new date of 18 April 2021.
