Christophe Euzet, MP for the Mediterranean port of Sète and lead sponsor of the bill, said he had been appalled at the way Castex, a former top civil servant and mayor of Pyrenean town Prades, had been mocked for his accent after taking the top government job.
Northern French often equate southern accents with sun, aperitifs and idleness.
Euzet, a native of Perpignan with similar southwestern tones to Castex, said it was time to end stereotypes in which a southerner is seen as "a fun guy ... not there to talk about serious things."
"Accents have no right of place in radio and television channels, in the world of politics and the helm of high office, administrations and French public businesses," he added.
Unlike in Britain, French media, in particular, has made no visible effort to employ newscasters and other prominent personalities with provincial twangs.
During debates, MPs complained that TV personalities with southwestern accents were "relegated to the rugby column or the weather".
Patricia Mirallès, a Macron MP, whose parents were French from North Africa, recounted "painful" memories of being mocked for her pied-noir accent.
Maina Sage, an MP from French Polynesia, denounced what she called "a form of racism" every time she opened her mouth in parliament.
"Our nation, which often prides itself on the great diversity of its regions, paradoxically disappoints through the toned-down uniformity of public speech," Euzet said.
The dominance of the Parisian accent is often equated with France notoriously centralised and urban administration whose failure to take into account the provincial mindset fuelled the "yellow vest" revolt two years ago.
Macron's appointment of Castex is seen as an attempt to show he is serious about wanting to hand more power back to the regions.
Half of French people say that they do not speak standard French, a notion that evolved from attempts launched in the 16th century to impose the accent of the Paris region around the country.
A quarter say they suffer mockery, according to an Ifop survey this year.
The worst victims of "accent discrimination" were residents of the banlieues - the often high-immigrant suburbs ringing French cities, along with the dialect of northern Picardy, known as "Ch'ti".
The culture clash between northern and southern accents and other stereotypes was the subject of Welcome to the Sticks, a 2008 comedy feature film that became one of the biggest Gallic box office hits in French history.
Euzet insisted the aim was not to outlaw jokes about accents but to "force a change of mentality".
The bill came a week after the "French accent" - regional or otherwise - lost its top spot as the world's sexiest.
In a global study carried out by Time Out in over 30 countries, the British accent took the top spot, crowned the world's sexiest with 25 per cent of votes, and the previous French champions were knocked into second on 16 per cent.
