Research on 140,000 men from eight European countries found that a 4in (10cm) larger waist circumference could increase the chances of getting the cancer by 13%.
Men were most at risk when their waist was bigger than 37in (94cm), the University of Oxford study found.
Prostate cancer is the most common cancer in men.
The study, which was presented at the European Obesity Summit in Gothenburg, Sweden, looked at the association between body measurements in men in their 50s and prostate cancer risk over 14 years.
In that time, there were about 7,000 cases of prostate cancer, of which 934 were fatal.
The researchers found that men with a higher body mass index (BMI) and waist circumference had an increased risk of high grade prostate cancer, an aggressive form of the disease.
For example, men with a waist size of 37in (94cm) had a 13% higher risk of aggressive prostate cancer than men with a waist of 33in (84cm).
Scientists also observed a higher risk of dying from prostate cancer with increasedBMI and increased waist circumference.
NHS Choices says there is a higher risk of health problems for men with a waist size of more than 94cm (37in) and for women of more than 80cm (31.5in).
Prostate cancer facts
- About 47,000 men are diagnosed with prostate cancer every year in the UK
- More than 10,800 men die from it every year in the UK
- One in eight men will get prostate cancer in their lifetime
- More than 330,000 men are living with or after prostate cancer
Source: Prostate Cancer UK
Dr Aurora Perez-Cornago from the Nuffield Department of Population Health at the University of Oxford said the study showed that the association between body size and prostate cancer was complex and varied by disease aggressiveness.
She said it was likely to be down to cancer-causing hormones in fat cells, but this had not yet been proven.
Her advice was that “men should try to maintain a healthy weight and if possible lose weight around their waist”.
But she added that the study had not specifically looked at the impact of losing weight on prostate cancer risk.
A spokesman for Prostate Cancer UK said: “Maintaining a healthy weight and staying active can protect against many diseases, including cancer.
“This research adds to a growing body of evidence that shows that weight and waist size could be another crucial risk factor for men to be aware of when it comes to protecting themselves against prostate cancer.”
Thea Cunningham, health information officer at Cancer Research UK, said more research was need to get a clearer picture of the link.
“It isn’t clear whether excess weight itself is causing men to develop aggressive prostate cancers, or if prostate cancers are less likely to be picked up at an early stage in overweight men, meaning their prostate cancer may be aggressive or advanced by the time it is diagnosed.
She added: “Keeping a healthy weight can help men reduce their risk of several other cancers including bowel cancer.”