The Metropolitan Police force said Liam Og O hAnnaidh, 27, was charged under the Terrorism Act with displaying a flag in support of a proscribed organisation.
The alleged offence happened at the Kentish Town Forum, a London concert venue, on November 21, 2024.
The force said the musician - whose stage name is Mo Chara and whom police referred to by the English spelling of his name, Liam O'Hanna - was charged by postal requisition and is due in court on June 18.
Police are still investigating footage from another Kneecap concert in November 2023.
The Belfast trio has been praised for invigorating the Irish-language cultural scene in Northern Ireland, where the status of the language remains a contested political issue in a society still split between British unionist and Irish nationalist communities.
It has also been criticised for lyrics laden with expletives and drug references and for political statements.
Kneecap in 2024 released a raucous feature film loosely based on the band's origins and fuelled by a heavy mix of drugs, sex, violence, politics and humour.
The group's members played themselves in the film Kneecap, which won an audience award at the Sundance Film Festival and a Bafta for outstanding debut.
It was shortlisted for best foreign-language picture and best original song at the 2025 Academy Awards, though it didn't make the final cut.
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