Helena Bonham Carter will leave the role next season.
The Crown has cast Mum star Lesley Manville as the Queen's sister.
She will be taking over from Helena Bonham Carter, who is currently in the role for seasons 3 and 4.
Lesley's version of Princess Margaret will continue into the 1980s for the fifth and final season.
This exciting news was announced on Twitter, with The Crown's official account sharing a statement.
They wrote, "Lesley Manville will play Princess Margaret in the fifth season of The Crown."
Lesley said, "I could not be happier to be playing Princess Margaret. The baton is being passed on from two formidable actresses and I really don’t want to let the side down.
"Furthermore, to play siblings with my dear friend Imelda Staunton will be nothing short of a complete joy."
The Crown has changed cast members to reflect the passing of time, with each key role being "aged up".
Queen Elizabeth was first played by Claire Foy, where we saw her marriage, coronation, and key events from early in her reign.
The baton was passed to Olivia Colman, who played the Queen as she experienced events such as the 1969 moon landing and the Aberfan disaster.
As for Princess Margaret, Lesley will be taking over from Vanessa Kirby and Helena Bonham Carter.
Lesley won't be on our screens until season 5, which hasn't yet filmed as a result of COVID-19.
Season 4 is expected to arrive at the end of the year, but a release date has not been confirmed.
However, we do know that there's some fantastic additions to the cast.
The X-Files star Gillian Anderson will join as the UK's first female Prime Minister, Margaret Thatcher.
And Emma Corrin will take on the role of Princess Diana.
The Crown seasons 1 - 3 are available on Netflix.
