During the BBC1 show tonight Lineker kept a straight face and didn't allude to the costume choice other than to say 'nothing much has really changed.'
Pundits and former England strikers Alan Shearer and Ian Wright could be seen giggling away as Lineker read out his introduction to the programme's first game after the Premier League got underway today.
Some on Twitter have criticised Lineker, 55, for opting for the large boxers, which featured a Leicester City logo, instead of the expected pair of skimpy briefs.
Fifteen minutes before the show started on BBC One, Lineker tweeted 'this is strangely terrifying'.
And fellow Match of the Day presenter Dan Walker joked: '4,000 sit ups a day for 20 seconds on telly. Well played Gary Lineker. Were they pants or shorts though?'
The show's first game was Hull City against Leicester, with the champions losing 2-1.
Lineker changed back to his usual smart shirt and trousers after 30 minutes in his pants.
His son George tweeted shortly after his father appeared in his pants. He had placed £50 on his dad wearing blue pants with odds at 4/5.
George Lineker tweeted: 'How the f*** did I get this wrong? I had inside info.'