In an analysis of six studies into fruit and vegetable intake, only food including spinach and cabbage was found to have a significant positive effect.
A portion and a half a day was found to cut type 2 diabetes risk by 14%, the British Medical Journal (BMJ) reports.
But experts urged people to continue to aim for five portions of fruit and vegetables a day.
This study suggests that green leafy vegetables seem to be particularly important in terms of preventing diabetes" Professor Melanie Davies University of Leicester
The researchers from Leicester University reviewed data from the studies of 220,000 adults in total.
They found that eating more fruit and vegetables in general was not strongly linked with a smaller chance of developing type 2 diabetes but "there was a general trend in that direction".
Yet when it came to green leafy vegetables, which the researchers said also includes broccoli and cauliflower, the risk reduction was significant.
The team calculated that a daily dose of 106g reduced the risk of diabetes by 14% - a UK "portion" is classed as 80g.
It is not clear why green leafy vegetables may have a protective effect but one reason may be they are high in antioxidants, such as vitamin C and another theory is that they contain high levels of magnesium.
Study leader Professor Melanie Davies, professor of diabetic medicine at the University of Leicester, said the message to eat five portions of fruit and vegetables a day remains an important one.
Diabetes UK is currently funding research into whether fermentable carbohydrates found in foods such as asparagus, garlic, chicory and Jerusalem artichokes could help weight loss and prevent Type 2 diabetes.