Mr Polanski had faced a challenge from local councillor Hamza Chowdhury to secure the nomination.
The Greens have picked party leader Zack Polanski as their candidate to challenge Labour in the Holborn and St Pancras seat vacated by Sir Keir Starmer.
Green Party MP Hannah Spencer declared him the candidate at an event in London, saying local party members “overwhelmingly” chose London Assembly member Mr Polanski.
Mr Polanski had faced a challenge from local councillor Hamza Chowdhury to secure the nomination.
He will take on Labour’s Sagal Abdi-Wali at the October 8 contest, triggered by former prime minister Sir Keir’s departure from Parliament.
The by-election could provide an early test of Prime Minister Andy Burnham’s popularity with the public.
Labour has held the Holborn and St Pancras seat continuously since it was created in 1983, but voters there have moved towards Green and independent candidates in recent years.
The Greens are likely to pose the greatest threat to Labour in the contest, but there is also much at stake for Mr Polanski’s party.
Support for the Greens surged in his first year as leader but the party’s national polling has dipped since Mr Burnham replaced Sir Keir in Downing Street.
Ms Abdi-Wali, who has led Camden Council since May this year and been a councillor for Camden Square since 2022, will seek to retain the seat for Labour.
Mr Burnham visited the constituency on Thursday to support Ms Abdi-Wali on her first day of campaigning, she said in an X post.
Only two Labour MPs have ever represented Holborn and St Pancras: former health secretary Frank Dobson, from 1983 to 2015, then Sir Keir.
At the most recent general election in 2024, Sir Keir held the seat with 48.9% of the vote, ahead of independent candidate Andrew Feinstein in second place on 18.9% and the Greens in third on 10.4%.
The Conservatives came fourth on 7.2%, Reform finished fifth on 6.1% and the Liberal Democrats were sixth on 5.8%.
The popularity of the independent and Green candidates at the 2024 contest, finishing second and third respectively, may have been partly down to former Labour voters protesting against Sir Keir’s refusal to take a stronger position on Israel’s actions in Gaza in the war with Hamas.
The Green Party also made gains in the May local elections in the London Borough of Camden, the local authority that covers the constituency.
Government chief whip Anneliese Midgley moved the writ in the House of Commons on Monday, officially triggering the by-election.
Sir Keir revealed his plans to leave Parliament earlier this month, saying he felt it was the “right time” to step down and focus on new work on international affairs.
