Blac Chyna -- Rob and I Have Our Own Song

Rob Kardashian's relationship with Blac Chyna has now been memorialized in song.

Blac Chyna posted video of a massive art piece she got from Rob ... emblazoned with lyrics from the couple's song, John Legend's "The Beginning."

And if that doesn't tickle your pickle ... he also gifted her more than 100 red roses.

By any other name, it's love.