The Equality Act defines disability as a physical or mental impairment that has a "substantial and long-term adverse effect" on the ability to carry out normal day-to-day activities.
Figures from the latest census of England and Wales show 17,979 people in Monmouthshire said they had such an impairment as of March 2021 – 17.7% of the area's population.
Of these people, 10,311 (10.3%) said their disability stopped them from carrying out regular activities 'a little', while 7,668 (7.3%) said it did so 'a lot'.
The overall proportion of disabled people is down from 2011, when 19.1% said they had a disability.
Across England and Wales, the proportion of people with a disability has fallen from 19.5% in 2011 to 17.8% at the last census – despite the number of disabled people increasing from 10 to 10.4 million.
But the ONS warned that the wording of the question was different in each census, with 2021 being the first to use the 2010 Equality Act definition of disability, and to explicitly mention mental impairments.
Census 2021 director Jon Wroth-Smith added that the "unique circumstances of the pandemic may have influenced the results".
The percentages used by the ONS have been standardised to account for differences in age between areas.
The latest census data also shows a quarter of households in England and Wales have at least one disabled member.
In Monmouthshire, there were 11,189 such households – including 2,786 with two or more disabled people.
Commenting on the figures, disability equality charity Scope said it was "high time" that society was more inclusive of those with disabilities.
Craig Moss, research manager at the charity, said: “Disabled people are repeatedly forgotten by government, business and society. Workplaces, pubs and public transport aren’t accessible.”
“Life costs a lot more, and disabled people have to fight to get the support they need,” he added.
The census also shows an improvement in the health of the populations of the two countries.
More people said their general health was 'very good' – 47.5%, compared to 45% in 2011, while the proportion saying it was 'very bad' dropped from 1.4% to 1.2%.
As of March 2021, 49.7% of Monmouthshire residents described their health as 'very good' – up from 47.5% in 2011.
Meanwhile, the proportion of people describing their health as 'very bad' fell from 1.3% to 1.1%.