Plaid Cymru and the Wales Green Party have agreed to form an electoral alliance in Caerdydd.
The two parties will draw up a joint slate of candidates to contest next year's council elections in the city.
The Plaid Cymru MS for Canol De Cymru Rhys ab Owen said that the alliance "can be the change that Cardiff needs to do politics in a new and more co-operative way".
Both parties have promised a single manifesto which pledges to give "real democratic power to communities", protect and enhance public green spaces, and provide fairer housing to the residents of Caerdydd.
Neither party is currently represented on Caerdydd council. Plaid Cymru had previously had four councillors, but are now longer represented on the council following defections.
Cymru's council elections will take place on 5 May 2022.