One in four children are overweight or obese, according to the 2018 Child Measurement Programme run by Public Health Wales.
The programme records the heights and weights of children in their reception year of primary school.
The results of the programme show the percentage of obese four and five-year-old children in Wales has increased over the past two years.
Merthyr Tydfil has highest prevalence of obesity where 17.5% of children are obese.
This is more than double the local authority area with the lowest prevalence, the Vale of Glamorgan at 7.8%.
Public Health Wales says the programme aims to help parents of under-fives to recognise whether their child is of a healthy weight. It is emphasising the importance of drinking less sugary drinks.