Speedo Mick, whose full name is Michael Cullen, is best known for walking the length and breadth of the UK for charity in his unique attire.
He has also swum the English Channel and climbed four mountains as part of his fundraising activity.
"I'm absolutely thrilled," Mr Cullen said, adding that the Citizen of Honour certificate would take pride of place in his front room.
He added: "I'm over the moon about it because it's from the city of Liverpool it makes it all the more special to me.
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Mr Cullen has raised money for over 120 organisations including charities which support disadvantaged young people and homeless people as well as mental health and wellbeing projects.
He said it was a way to thank the people who helped him with his own mental health challenges.