'Love and Hip Hop: Atlanta' Sex Tape Star Nikko Smith -- I Penned a Song About My Rod

"Love and Hip Hop: Atlanta" star Nikko Smith is so in love with his suddenly famous junk ... he wrote a song to celebrate it -- and TMZ has audio of the phallic ode.

As we first reported ... Nikko and Mimi Faust's very dirty shower work in "Mimi and Nikko: Scandal in Atlanta" has been raking in major dough -- and it also inspired Nikko to write "New Shower Rod."

The song's an instant Valentine's Day classic with lyrics like ... "Don't you change a thing, my personal freak" -- and "Let me f*** you in the shower, hands up on the rod."