In joining Alba, she became the party’s first sitting member of the Scottish Parliament.
She stood for leadership of the party in March this year, following the sudden death of founder and former first minister Salmond in October 2024.
However, she was narrowly defeated by MacAskill in the ballot.
A spokesperson for the party said it was “not unexpected” that she had chosen to leave the party having not entered selection for next year’s vote.
They added she had “not taken an active role” within the party since the leadership vote.
The party said: “Ash has rightly earned respect within and beyond the Alba Party for the fearless way in which she has championed the rights of women.
“The Alba Party thanks her for her contribution in the Scottish Parliament on behalf of the party.
“We wish her every personal happiness for the future.”
