The 22-year-old had only signed a contract extension at Peterborough until 2030 in April but now join Millwall on a "long-term" deal.
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Lisbie scored 14 goals in 51 games in all competitions last season after making the step up to League One from non-league Braintree Town back in 2025.
Millwall director of football Steve Gallen said the club initially tried to sign the winger during the January transfer window.
"Kyrell had a real breakout season last year at Peterborough, with an excellent goals and assists record that speaks for itself," Gallen told the club website.
"He can play left or right-wing, he's quick, direct, and he's got a real hunger to score goals and create chances."