Researchers studied 160 patients being treated for heart failure using a variety of proven drug treatments and pacemakers.
Participants who took a vitamin D3 supplement daily for one year experienced an improvement in heart function not seen in those given a dummy pill.
Lead scientist Dr Klaus Witte, from the School of Medicine at Leeds Teaching Hospitals NHS Trust, said: "This is a significant breakthrough for patients.
"It is the first evidence that vitamin D3 can improve heart function of people with heart muscle weakness, known as heart failure. These findings could make a significant difference to the care of heart failure patients."
The researchers measured "ejection fraction", which shows how much blood is pumped from the heart with each heartbeat.
A healthy person's ejection fraction is usually between 60 per cent and 70 per cent.
Among the heart failure patients taking part in the study it averaged 26 per cent, but increased to 34 per cent for those taking vitamin D.
For some patients, regularly taking vitamin D might reduce the need for an implantable cardioverter defibrillator (ICD), a device that corrects abnormal heart rhythms, said the scientists whose findings were presented at an American College of Cardiology meeting in Chicago.
Heart failure, which can lead to shortness of breath and exhaustion, affects about 900,000 people in the UK and more than 23 million worldwide.
Vitamin D levels are boosted by exposing the skin to sunlight. It is often lacking in heart failure patients because they tend to be older and less likely to engage in outdoor activities.
Comment: Other studies have confirmed that low levels of Vitamin D have been linked to many diseases including cancer, high blood pressure, heart disease, diabetes, depression, fibromyalgia, chronic muscle pain, bone loss, and autoimmune diseases like multiple sclerosis. Due its absence in our food supply and the phobia against sun exposure, most people are deficient. It is also important to know that if your levels remain low despite taking vitamin D supplements, a magnesium deficiency may be one of the reasons you can't correct it. Supplementing with magnesium may be necessary because it is quite difficult to obtain enough from food sources as our soils have been deficient in magnesium for decades, and like Vitamin D it is essential for our health.