Sir Keir donned pristine red gloves sporting the logo "vote Labour" as he was shown the ropes at the Vulcan Boxing Club in the city on Friday morning.
The leader of the Opposition took off his jacket and began giving a red punchbag everything he had, joking: "I'm going to be wearing these at Prime Minister's Question Time."
Warming to the idea as he posed for pictures, Sir Keir held up the gloves and said: "Walking into PMQs."
And, as he aimed more heavy blows at the bag, he shouted: "My next question....and finally...."
He said: "One day, I've got to do it."
Seb Glazer, who runs the Vulcan Centre where the boxing club meets, joked with Sir Keir about the prospect of a pugilistic Commons confrontation.
The Labour leader laughed, suggesting: "The last question is from Seb. Fantastic."
Mr Glazer suggested Sir Keir watch professional Hull cruiserweight Sonny Taylor take aim at another bag. The politician, still wearing his white shirt and suit trousers, said he was "similar".
He confessed to Mr Glazer that he has no experience of boxing and laughed when he was told he was not allowed to punch anyone due to Covid regulations.
Sir Keir toured the Vulcan Centre, which he heard had become a beacon for the community during the pandemic - offering sporting activities, training and one of the largest food banks in Hull.
He later said that the wide availability of the Prime Minister's mobile number represents "a security risk".
Chancellor Rishi Sunak played down fears that Boris Johnson had opened himself up to possible covert activity by hostile states and exploitation by criminal gangs after it emerged that his personal contact details had been accessible on the internet for the past 15 years.
Sir Keir said: "He was warned about it - that tells its own story. I think a lot of people will be concerned, not just about who's got the number but who's been using it.
"I changed my number when I became director of public prosecutions, and I've kept my number secure since then."