"After a period of serious reflection, I have taken the following decision: I want to be the next mayor of Barcelona," the former Socialist premier said in Catalan at an event in the old centre of Barcelona, Spain's second biggest city where he was born 56 years ago.
"Since my birth... my relationship with Barcelona has been intimate, constant," added Valls, who grew up in Paris with his Catalan father and Swiss-Italian mother and was nationalised as a French citizen he was 20.
Under European Union rules citizens of the bloc can vote and stand in local elections in other EU states.
But beating Ada Colau, a former housing activist who is the current mayor of the city of 1.6 million, in the municipal elections slated for May 26 next year will be a challenge.
Valls has campaigned in Spain against Catalan separatists who attempted to break away from the country last October.
During his speech in both Catalan and Spanish to a packed auditorium on Tuesday, Valls listed a series of problems facing the city: rising insecurity, tourism saturation, lack of affordable housing and the flight of companies due to concerns over Catalonia's separatist push.
"The city is plunged into a losing dynamic, we have to stop it, we need to change course and a new leadership as soon as possible," Valls said.
Valls will have the support of centre-right party Ciudadanos, which is spearheading opposition to the independence drive in Catalonia.
He has recruited a former communications director of FC Barcelona, the giant football club he supports, for his campaign.
But "his chances of becoming mayor are slim," said Jordi Munoz, politics lecturer at the University of Barcelona.
Valls's firm stance on public order, as shown in France when he was interior minister, may not go down well in the traditionally leftwing city.
But Joaquim Coll, a historian and political analyst who is close to Valls, said his candidacy would "shake up" the political landscape, adding it was a "risky" but not "suicidal" bid.
Catalonia's former pro-independence president Carles Puigdemont was dismissive of Valls.
"He's a candidate who doesn't know Barcelona, who's not known in Barcelona," Puigdemont said in Brussels, where he is currently exiled.
Valls has been criticised in France for ditching the Socialist Party after losing out in the 2017 party presidential primary.
He failed to join forces with the winner of the election, Emmanuel Macron, and to enter parliament in Macron's centrist grouping.
Valls said he would step down as a lawmaker in the French parliament as of next week and had informed Macron of his plan to run for mayor of Barcelona "this summer".
"I don't know what he's coming here for," said Laura Bozzo, a retiree, in front of Barcelona's city hall.
"I reckon that as no one wants him in France, he's coming to Barcelona."
Bank employee David Centellas disagreed.
"He's a prestigious person, with international recognition and he can improve Barcelona's image."