The research team calculated the amount of fat they had - their Body Mass Index (BMI) - from their height and weight.
Anything over 25 is overweight and over 30 is obese, reports the Sun.
From the analysis, the team found that men with a BMI of 32 were least likely to father a child - while lower levels of sex hormone testosterone were found in every overweight man.
Two other key fertility hormones - luteinizing and follicle-stimulating - were also depleted.
The finding implies that the testicles, where sperm is produced, are not working properly.
Head researcher Dr Eric Pauli, of Pennsylvania State University College of Medicine, said couples being assessed for unexplained infertility should take the weight of both partners into account.