The singer is being sued for copyright infringement by Tempo Music Investments, a company claiming it owns a share of the copyright for Bruno's song "When I Was Your Man."

In the lawsuit, obtained by TMZ, Tempo Music claims there are so many musical similarities between Miley's "Flowers" and Bruno's "When I Was Your Man" that it's obviously the result of Miley "intentionally copying" Bruno.

Tempo Music claims the melody, harmony and chorus in "Flowers" are substantially similar to the verse in "When I Was Your Man" ... and, they say Miley's song copies the chord progressions and mirrors some of the lyrics too.

Miley won a Grammy Award for "Flowers" ... but Tempo Music says she wouldn't be getting her flowers without the Bruno song, which was released a decade earlier and reached the top of the charts.

In the docs, Tempo Music says ... "It is undeniable based on the combination and number of similarities between the two recordings that ‘Flowers’ would not exist without ‘When I Was Your Man.’”

Tempo Music says Miley, her label and her producers all had access to Bruno's song ... and they're going after Miley for money, and they want her to be barred from distributing or performing "Flowers."

Worth noting ... Bruno Mars is not a plaintiff in the lawsuit.