All diets - from Atkins to Weight Watchers - have similar results and people should simply pick the one they find easiest, say researchers.
The study, published in the Journal of the American Medical Association, analysed data from 48 separate trials.
The Canadian team concluded that sticking to a diet was more important than the diet itself.
Obesity experts said all diets cut calories to a similar level, which may explain the results.
Diets go in and out of fashion on a regular basis, with a current debate around the relative benefits of low carb and low fat diets.
All the same
Scientists at McMaster University in Ontario and the Hospital for Sick Children Research Institute in Toronto analysed data from 7,286 overweight dieters.
The range of diets covered included, Atkins, South Beach, Zone, Biggest Loser, Jenny Craig, Nutrisystem, Volumetrics, Weight Watchers, Ornish and Rosemary Conley.
It showed that after 12 months, people on low carbohydrate and low fat diets both lost an average of 7.3kg (16lb). Those on low carb meal plans had lost slightly more at the six-month marker.
The report said: "The differences [between diets] were small and unlikely to be important to those seeking weight loss."
It concluded: "Our findings should be reassuring to clinicians and the public that there is no need for a one-size-fits-all approach to dieting because many different diets appear to offer considerable weight loss benefits.
"Our findings suggest that patients may choose, among those associated with the largest weight loss, the diet that gives them the least challenges with adherence."
However, the study did not look at wider health issues, such as levels of cholesterol, which may vary according to diet.
Prof Susan Jebb, from the University of Oxford and a government advisor on obesity, said diets were more similar than they appeared, advocating cutting calories to 1,500 a day, sticking to strict meal times and avoiding biscuits, cakes and chocolate.
"The issue is about adherence and it's how closely and how long can you keep sticking to the plan over time that matters.
"That probably means finding the right diet for you, rather than one being so particularly better than the others."
She said people should try to match diets to their lives.
Vegetarians would struggle more with a high protein, low carb diet, while people living on their own may find liquid (instead of meals) diets easier than those who would still have to cook for a family.