The study found youngsters with high blood pressure are more likely to struggle at school because the condition reduces brainpower.
High blood pressure, also known as hypertension, was once mainly seen just in adults.
But now the condition also affects one in 20 children thanks to bad diets, excess weight and not enough exercise.
One in three children is overweight or obese by the time they leave primary school in the UK.
High blood pressure has previously been linked with a decline in an adult's cognitive function.
But the new study is one of the first to discover the phenomenon in children and teenagers.
The study of 10 to 18-year-olds found those with hypertension performed worse on tests that measured visual and verbal memory, processing speed and language skills.
Researchers compared 75 newly-diagnosed participants with a control group of the same number of healthy children matched for age.
The findings, published in The Journal of Pediatrics, also showed more children with sleep issues had hypertension - which intensified the effect of poor sleep on cognition and executive function.
Children who had other factors that are known to affect cognitive skills - such as ADHD (attention-deficit-hyperactivity-disorder), learning disabilities and sleep disorders - were excluded.
Dr Marc Lande, of Rochester University in New York, said: "We wanted to make sure if we found differences between children with and without hypertension it was likely associated with the hypertension itself - not any of these other factors."
Overall the study provides evidence high blood pressure in children is associated with a subtle pattern of decreased cognitive performance.
Dr Lande said: "In the future we want to better understand if there are physical changes to the brain in children who have hypertension that could explain these cognitive test results."
Knowing how these physical changes might affect cognitive skills could be important in future studies assessing if high blood pressure pills could improve academic performance - and reverse or prevent future adult hypertension-related problems.
Dr Lande said it's important to note the differences between groups were small and the average cognitive test scores were largely within normal ranges.
The children with hypertension were not cognitively impaired - but rather performing less well than children without the condition.
Children who are obese are up to six times more at risk of high blood pressure, which is commonly linked to heart disease in later life.
Young obese girls have the biggest problem, with a 5.9-fold chance of developing it compared with those of healthy weight while the risk is four times greater for obese boys.
Ten years ago a study by French scientists found adults with a BMI (Body Mass Index) of 20 or less could recall 56 per cent of words in a vocabulary test while those who were obese with a BMI of 30 or higher could remember only 44 per cent.
Another US study found children showing physical changes due to being obese - such as raised blood pressure, higher levels of bad cholesterol and resistance to the blood sugar controlling hormone insulin - had poorer scores on thinking tests.