The Internet's abuzz with this surprising Britney sighting last night ... at the premiere of the new Brad Pitt-Leonardo DiCaprio-Quentin Tarantino film Once Upon A Time In Hollywood! Spears walked the red carpet -- something SO rare for her -- with boyfriend Sam Asghari, and these two looked (literally) red hot!

Britney was gorgeous in a bright red dress and diamond necklace with her blonde locks pulled back. Sam looked so debonaire in a cream-colored suit. The couple looked old-Hollywood glam, in keeping with the film's theme.

Of course the film's stars didn't go unnoticed (though they were perhaps slightly upstaged by Brit) -- Brad was beaming, as this was his official return to the big screen post-Angelina Jolie breakup. Leo was his usual cool self, waving to fans, signing some autographs, but all with a tone of restraint.

Once Upon A Time In Hollywood, according to the film's web site, "visits 1969 Los Angeles, where everything is changing, as TV star Rick Dalton (Leonardo DiCaprio) and his longtime stunt double Cliff Booth (Brad Pitt) make their way around an industry they hardly recognize anymore. The ninth film from the writer-director features a large ensemble cast and multiple storylines in a tribute to the final moments of Hollywood’s golden age." Ummm ... seeing it!