Pompey left it late before seeing off AFC Wimbledon to move up one place to ninth in the League One standings.
Danny Cowley's side had to battle from behind after Ollie Palmer converted an extremely dubious penalty that was won by Nesta Guinness-Walker.
But Michael Jacobs' first goal of the season drew them level and Marcus Harness fired home in the closing stages to seal a 2-1 victory at Fratton Park.
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