The record number provides the clearest indication yet of the scale of the obesity problem as coronavirus started to spread across the UK.
Being overweight is one of the most significant risk factors for severe Covid.
Experts say the data should be a wake-up call for tackling obesity.
The figures, published by NHS Digital, show a 17% increase in hospital admissions where obesity was a factor, compared with the year before.
This amounts to almost 150,000 more instances of people being admitted to hospital over the course of a year.
The number of admissions where obesity was recorded as the main cause actually fell to 10,780 last year, from 11,117 in 2018-19.
But this is mainly due to a fall in the number of bariatric surgeries (including gastric bands and bypasses) being performed.
Women accounted for two-thirds (64%) of admissions where obesity was a factor.
Naveed Sattar, professor of metabolic medicine, at the University of Glasgow, says obesity is "the strongest risk factor for [Type 2] diabetes".
"It's a strong risk factor for heart disease, for heart failure, for lung disease, for kidney disease, for multiple other conditions," he says.
"If we now add to that obesity is a strong risk factor for this acute viral pandemic which is killing people worldwide, then... this is a wake-up call.
"Many health systems and government really do now need to pay attention to obesity.
"We do need to tackle obesity and we need to take it seriously."
'They told me I could die'
At his heaviest, Phil Skeates weighed 25 stone (159kg).
And when he developed coronavirus, last year, he ended up needing intensive care.
"It was horrible," Mr Skeates says.
"I was blue-lighted.
"As I was taken out of the house, I said goodbye to my wife.
"I was crying my eyes out in the ambulance.
"It was heartbreaking.
"My thoughts were I may not see her again.
"They'd actually told me if I caught Covid, there was every chance that I would die."
"On the ward, I saw two people pass away.
"It was a massive massive point.
"It really made me think I need to address this."
Phil has now lost more than six stone.
"I paid the price," he says.
"I've come through it and I'm not going back."
Scientists are still trying to understand why the coronavirus poses such a risk to those significantly overweight.
Prof Sattar says there is some evidence heavier people have a higher viral load or more virus in their bodies than others and the virus triggers a dangerous response by the immune system.
"There may be a critical interaction between fat cells and the immune response which increases the likelihood of that immune response being exaggerated and harmful," he says.
"We also know that people who are heavier have thicker blood to begin with, and this hyper-response thickens the blood even more.
"So the likelihood is that this thickness will clot off blood vessels and block blood vessels.
"People who are overweight, effectively have less capacity to deal with the damage Covid causes."
And for many people, the pandemic has led to weight gain, early evidence suggests.
NHS England medical director Professor Stephen Powis said: "Today's shocking figures are a growing sign of the nation's obesity crisis which is putting hundreds of thousands of people at greater risk of becoming severely ill with Covid, as well as heart attacks, stroke, cancer and other deadly diseases".
This is a real concern, Susan Jebb, professor of diet and population, at Oxford University, says.
"People have gained 2-3kg but that is very unevenly spread - some people have gained much more," she says.
"And it seems to be a particular problem for women, for younger people and for those living in deprived areas."
Pressure is piling on the government to take radical action to address the obesity problem.
Last month, doctors, academics, campaigners and public-sector experts wrote to the prime minister, urging him to stick to a landmark government proposal to ban junk-food advertising online and on social media - after fears it could be ditched.
The government says it has a clear plan to introduce a ban on junk-food adverts on television before 21:00.
It has now finished a consultation with industry and how far it is prepared to go with restrictions online will be revealed in the next few weeks.