The 39-year-old learned the news after taking tests during his Disney+ documentary series Limitless.
Hemsworth's higher risk of the disease is down to genetic factors - he has two copies of the gene ApoE4, which he inherited from both of his parents.
In an interview with Vanity Fair, The Thor actor shared that the tests confirmed his 'biggest fear' and added he will now aim to take 'preventative steps'.
They showed he is part of the 2-3% of the population who are between eight and 10 times more likely to develop the disease than those without two copies of the gene.
Explaining his decision to take a break from acting, Hemsworth began by telling the magazine: 'It’s not like I’ve been handed my resignation.'
He said, however, that working on Limitless, which sees him challenge his body and learn more about wellbeing 'really triggered something in me to want to take some time off'.
Hemsworth continued: 'If you look at Alzheimer’s prevention, the benefit of preventative steps is that it affects the rest of your life.
'It’s all about sleep management, stress management, nutrition, movement, fitness. It’s all kind of the same tools that need to be applied in a consistent way.'
The father-of-three clarified that he's not been diagnosed with Alzheimer’s disease, but has been warned about his heightened risk.
'It’s not a pre-deterministic gene, but it is a strong indication. Ten years ago, I think it was more thought of as determinant,' he said.
Hemsworth explained that the original plan for the series was for him to receive all his genetic test results live, but series creator Darren Aronofsky told him privately once they got the results.
The Marvel star was given the option to remove any references to Alzheimer's from the documentary, but chose to include his genetic risk, as a way of raising awareness and increasing understanding.
He said: 'My concern was I just didn't want to manipulate it and over-dramatise it, and make it into some sort of hokey grab at empathy or whatever for entertainment.'
Hemsworth also confirmed to the same publication that his grandfather has been diagnosed with the disease.
The actor will be seen in George Miller's forthcoming Mad Max sequel Furiosa, which wrapped filming earlier this month.
He clarified that he'll take an acting break after finishing his publicity tour for Limitless, in addition to his other contracted work.
Hemsworth will be returning home to Byron Bay in Australia to be with his wife-of-12-years, actor Elsa Pataky, and their children, 10-year-old daughter India Rose and twin sons, Tristan and Sasha, eight.
Alzheimer's disease is the most common cause of dementia in the UK and typically affects people over the age of 65, according to the NHS, however one in every 20 people with the condition are under that age.
It's a progressive disease, meaning symptoms develop gradually over many years and eventually become more severe.
The first sign of the disease, which affects multiple brain functions, is usually minor memory problems.
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