Colau, one of the leading lights of the Indignados (Outraged) protest movement that grew out of Spain's economic crisis, will be the Catalonian city's first female mayor.
Her list of candidates won 11 of the 41 seats in local elections on May 24 that saw the Indignados hammer Spain's ruling conservatives. Her nearest rival, the incumbent conservative Xavier Trias, won 10 seats.
But her position as mayor was secured when the ERC, a left-wing independent party that has five seats and the Socialists, who have four seats, threw their support behind her.
“We are going to vote for Ada Colau to be sworn in through a sense of responsibility because we believe in the union of people committed to social justice, progress and wellbeing,” said ERC leader Alfred Bosch.
Colau, 41, has pledged to fight inequalities in the city of 1.6 million by putting an end to evictions from housing, lowering energy prices and bringing in a minimum monthly income of 600 euros ($675).
Spain's capital Madrid will also have a leftist mayor on Saturday after 71-year-old retired judge Manuela Carmena's Ahora Madrid protest party agreed an alliance with the Socialists. Carmena has also made ending evictions a priority in a bid to reduce homelessness.