Johnson was photographed running by Marks and Spencer in London with his excitable Jack Russell.
The Prime Minister, dressed in one of his array of woollen hats, recently announced he has lost a stone from running and ditching 'late night cheese'.
He has been on a fitness drive since his Covid battle last year.
Johnson famously admitted 'I was too fat' after the Covid scare that saw him taken into hospital — and later intensive care — with the virus last April.
Boris Johnson runs with his dog Dilyn in central London this morning with his security in tow behind
The Prime Minister visited the shop before running with his security team in tow behind.
He wore a face mask, jumper and shorts, braving the early morning temperatures of just 37.4F (3C).
It comes after Johnson yesterday led tributes to the late Prince Philip, hailing the Duke of Edinburgh for his 'extraordinary life and work'.
He said: 'It is to Her Majesty, and her family, that our nation's thoughts must turn today.
Boris Johnson leaves Marks and Spencers in central London
'Because they have lost not just a much-loved and highly respected public figure, but a devoted husband and a proud and loving father, grandfather and, in recent years, great-grandfather.
'Speaking on their golden wedding anniversary, Her Majesty said that our country owed her husband 'a greater debt than he would ever claim or we shall ever know' and I am sure that estimate is correct.
'So we mourn today with Her Majesty The Queen we offer our condolences to her and to all her family and we give thanks, as a nation and a Kingdom, for the extraordinary life and work of Prince Philip, Duke of Edinburgh.'
Matt Hancock yesterday said tackling the nation's obesity levels is a 'particular obsession' of Boris Johnson after his brush with Covid.
Speaking at the launch of the All Party Parliamentary Group for Longevity's Levelling Up Health report, the Health Secretary said that 'improving the disparities in healthy life expectancy is absolutely at the core of our levelling-up agenda'.
Earlier this year, the Prime Minister pledged to tackle Britain's obesity crisis following his admission to intensive care with coronavirus.
Mr Hancock said 'healthy life is unevenly distributed across the country' and issues around food, obesity, smoking, air pollution and the chances of children should be tackled.
He said: 'We know that smoking is the single biggest preventable killer in this country, and that the disparities in smoking rates are still far too wide.
'The estimate is that around half the gap in healthy life expectancy can be explained and therefore resolved by differences in the smoking rates.
'Obesity is clearly critical, and has had a significant impact on people's morbidity and mortality when hit by Covid, and is a particular obsession of the Prime Minister's after his experience with Covid.
'And I have to say he's looking absolutely fantastic, and has clearly lost quite a lot of weight in the last few months, and I hope that we can encourage the nation to too.'
Mr Hancock said he wanted to incentivise the whole system so that people do not end up in 'expensive care' in hospitals.
He said putting more funding into the community 'based on this concept of population health' could help keep people out of hospital.
Mr Hancock said 'prevention being better than cure' needed to be a principle 'that runs through every decision, every allocation of resources across the NHS'.