A gene associated with obesity also appears to reduce a person's risk of depression by eight percent.
Researchers from McMaster University say the FTO gene is associated with both obesity and happiness.
"We [started] from the hypothesis that both depression and obesity deal with brain activity. We hypothesised that obesity genes may be linked to depression," study author David Meyre said.
They examined the genetic and psychiatric stats of 17,200 DNA samples from people from 21 countries.
They found a genetic alteration to the FTO gene that makes people vulnerable to obesity could also be linked with an eight percent reduced chance of developing depression.
"The difference of eight percent is modest and it won't make a big difference in the day-to-day care of patients. But we have discovered a novel molecular basis for depression," Meyer said.
Associate Professor John Dixon, from Monash University and head of clinical obesity research at Baker IDI Heart and Diabetes Research Institute, told ninemsn that there is a strong link between obesity and depression and that one genetic marker is not likely to make a difference.
"These genetic associations are fascinating but the clinical messages are a long way off," he said.
"The levels of depression get to be as high as 40 percent in those that are severely obese. We think it's related to conditions such as diabetes, sleep disturbance, the psychological issues of being very big and the chemicals related to being severely obese. It's not as simple as a single gene."
Associate Professor Dixon said obesity is could be related to how our genes are programmed in early life.
"It's not just our genes, it's how our genes interact with the environment," he said.
"The message that obesity is associated with happiness is very much the wrong one because that's not the situation in our community."
The study was published in the journal Molecular Psychiatry.