The University of Pittsburgh study found the 55-minute regime was the minimum needed to maintain a 10% drop in weight.
Only a quarter of the 200 women in the study managed to lose this amount.
A UK expert said it was clear that regular moderate exercise was the way to lose weight, and keep it off.
Approximately two-thirds of adults in the UK are overweight or obese, with some estimates suggesting this could rise to nine in ten by 2050.
Research points to a combination of exercise and calorie control as having the best chance of success in weight loss - although the majority of people who attempt these diets fail to keep the weight off.
The latest research, published in the Archives of Internal Medicine journal, confirms that plenty of exercise is a key ingredient of success.
The Pittsburgh researchers looked at a group of overweight and obese women over a four year period.
They were all told to eat between 1,200 and 1,500 calories a day, and split into four different exercise programmes, varying the intensity and amount of exercise carried out.
After six months, women in all four groups had lost up to 10% of their body weight - but most could not keep this going.
The women who did maintain the 10% loss were those who reported doing more exercise, on average 275 minutes per week.
Hour target
In the UK, everyone is encouraged to manage at least 30 minutes of moderate exercise five days a week, but scientists say that people wanting to maintain weight loss need much more.
Professor Paul Gately, a specialist in exercise and obesity, from Leeds Metropolitan University, said: "This study is excellent - it's the best evidence that this higher level of exercise is needed.
"Thirty minutes a day is good for general health, but if you want to lose weight, you need to be doing more, and if you want to sustain weight loss, you need to be doing even more than that.
"In this country, between an hour and 90 minutes of exercise a day is the recommended level for maintaining weight loss."
He said that "moderate" exercise covered anything that made you slightly breathless, from brisk walking to gardening.
Some US researchers have been looking at novel ways to increase the level of activity among those with "couch potato" lifestyles.
Dr James Levine is helping develop the "Walkstation", a computer terminal with a treadmill instead of a chair.
Writing in the same journal, he said: "Sitting at one's desk uses barely any more calories than staying in bed, but walking at 1mph while working increases caloric expenditure by approximately 125 calories an hour."
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