A five year study of more than 3,000 middle aged and elderly people in Norfolk found those who had a dog were 20 per cent more active - when the weather was at its worst.
Scientists say taking Fido walkies is key to staying healthy in later life - reducing the risk of heart disease, cancer and even dementia.
It it lopped an average of 30 minutes off the time they spent sitting each day - and the greatest benefit was during winter.
Regular dog walkers did more activity when the thermometer dropped below 10 degrees Celsius than petless counterparts managed - on the warmest days!
On shorter days and those that were colder and wetter all the participants tended to be less physically active and spent more time sitting but dog owners still went out.
Project lead Professor Andy Jones, of the University of East Anglia, said it suggests dog ownership or community schemes for dog walking could form part of 'exercise on prescription'.
He said: "We were amazed to find dog walkers were on average more physically active and spent less time sitting on the coldest, wettest, and darkest days than non-dog owners were on long, sunny, and warm summer days.
"The size of the difference we observed between these groups was much larger than we typically find for interventions such as group physical activity sessions that are often used to help people remain active."
Almost one in five of the sample (18%) said they owned a dog and two thirds walked it at least once a day so were classified as regular dog walkers.
The entire sample spent an average of around 11 hours every day sitting and tended to be less active when it rained, was cold and the days were short.
But during cold wet winters exercise levels of dog owners were typically 20 per cent higher.
Fewer than half older Britons get the recommended weekly 150 minutes of moderate intensity physical activity.
Research has shown regular brisk walks is good for mental - as well as physical - health by boosting blood flow to the brain.
Lead author Dr Yu-Tzu Wu added: "We know physical activity levels decline as we age but we're less sure about the most effective things we can do to help people maintain their activity as they get older.
"We found dog walkers were much more physically active and spent less time sitting overall. We expected this.
"But when we looked at how the amount of physical activity participants undertook each day varied by weather conditions we were really surprised at the size of the differences between those who walked dogs and the rest of the study participants."
The study showed owning or walking a dog was one of the most effective ways to beat the usual decline in later-life activity - even combating the effects of bad weather.
Dog owners were sedentary an average 30 minutes less per day, according to the findings published in the Journal of Epidemiology and Community Health.
The study involved 3,123 people aged 49 to 91 in the long-running EPIC Norfolk cohort study who wore a pedometer round their waist that measured physical activity over seven consecutive days during waking hours.
They also provided information on regular exercise.
Bad weather and short days are known to be one of the biggest barriers to staying active outdoors so researchers linked walking to weather conditions and season on each day of the study.
Prof Jones said: "Physical activity interventions typically try and support people to be active by focusing on the benefits to themselves but dog walking is also driven by the needs of the animal.
"Being driven by something other than our own needs might be a really potent motivator and we need to find ways of tapping into it when designing exercise interventions in the future."
Borrow my Doggy, a nationwide UK network, which provides regular group dog walks for people who aren't dog owners, might be one such option, they suggest.
The findings follow a similar study by Glasgow Caledonian University last month that found pensioners who take their four legged friends out daily walk an extra 22 minutes per day compared to those who don't own a pet.
The Scottish team said GPs should be encouraged to suggest older people get a dog or share one to encourage them to walk more.