Kendrick Lamar Sued -- We Got Stiffed By 'Rigamortis' ... Songwriters Claim

Kendrick Lamar is a musical thief who blatantly ripped off another song to create his hit, "Rigamortis" ... at least according to two musicians who are suing him for at least $1 million.

Eric Reed and Willie Jones III claim they came up with a song called "The Thorn" in 2010 that features an infectious rhythm and commanding horns ... the very same music heard in Lamar's "Rigamortis."

In their lawsuit ... they say Lamar took their music, and simply added his own lyrics. They say "The Thorn" isn't merely part of "Rigamortis" ... IT IS "Rigamortis."

Reed and Jones say even though "Rigamortis" basically made Lamar a star ... he never gave them a dime or even credit for the music.

They're now suing Lamar for at least a mil, plus all 'Rigamortis' profits and rights to the song.

So we gotta ask ...