Although many would associate the fruit with summer months, researchers found that the watermelon's properties work throughout the year - even in the winter months when people are most at risk of heart attack.
In cold temperatures, the body is forced to work harder to stay warm. This increases blood pressure and increases the risk of heart attack or stroke.
But researchers found that eating watermelon reduced the risk of heart problems.
Professor Arturo Figueroa, from Florida State University, said: "The pressure on the aorta and on the heart decreased after consuming watermelon extract."
The 12-week study focused on 13 middle-aged obese men and women with high blood pressure. During the study period participants were asked to stop taking any medication for blood pressure.
To simulate cold weather conditions, researchers asked participants to dip their hand into water measuring 4 degrees Celsius.
Half of participants were given watermelon extracts - four grams of amino acid L-citrulline and two grams of L-arginine - to consume every day. and the other half were given a placebo. After six weeks the two groups unknowingly swapped roles.
According to researchers, watermelon was found to lower blood pressure and reduce stress on the heart.
"That means less overload to the heart, so the heart is going to work easily during a stressful situation such as cold exposure," said Professor Figueroa.
The study was published in the American Journal of Hypertension.