Eight Basque Surnames (Ocho Apellidos Vascos) by director Emilio Martínez Lázaro "is the most watched Spanish film with 6.6 million viewers" in cinemas since its release on March 14th, Universal Pictures Spain said in a statement.
Playing with regional stereotypes and Spain's north–south divide, the film treads sensitive ground with the tale of a red-blooded Rafa from Andalusia in Spain's deep south, who comically pretends to be a proud Basque in his efforts to woo a Basque woman, Amaia, on her home turf.
Cautious hopes have arisen in the Basque country after the armed separatist movement Eta in 2011 promised a definitive end to decades of violence there.
Eta is blamed for more than 800 deaths in its armed independence campaign. It is classed as a terrorist group by the United States and the European Union.
"If Eta were still killing, this film could not have been made," the director was quoted as saying by El País newspaper.
The film has drawn praise for tackling sensitivities about Basque identity and cultural difference in Spain with a light-hearted touch.
"It does away once and for all with the most alienating stereotypes, at least the ones that have done the most harm," wrote journalist Juan Mari Gastaca in El País newspaper.
Not all coverage of the film has been positive, however.
An earlier review in El País by Jordi Costa criticized the film's poor direction and plot inconsistencies, describing the movie as an "excellent opportunity missed".
The film beat the previous record for the number of tickets sold, set by Spanish director Alejandro Amenabar's 2001 horror film "The Others", starring Nicole Kidman.
"Eight Basque Surnames" has taken €38.8 million ($54 million) at the box office, the fourth-biggest takings ever in Spain after "Avatar", "The Impossible" and "Titanic", Universal said.