The singer-songwriter went topless on Instagram to show off the new ink, which has been done entirely in white.
A swirling, symmetrical pattern now covers her entire back, stretching up to her shoulder-blades.
Grimes’ pink streaked hair falls down her back as she leans on a pillow to give fans a good look at the new tats.
“Full back in white ink by @tweakt w sum help from the alien computer brain of @nusi_quero” she captioned the pic.
Grimes added: “Don’t have a good pic cuz it hurts too much and I need to sleep haha, and it’ll be red for a few wks, but gna be beautiful alien scars.
Followers were quick to flood the comments section, with one writing: “Unreal”.
Someone else wrote: “Beautiful alien momma”.
And a third fan wrote: “Can we please see it under blacklight?”
Grimes and Elon started dating in 2018.
Others weren’t so sure about the painful-looking design, with one adding: “Omg your ribs must be hurting so much! Take care!”
Another social media user typed: “This looks painful to get.”
The Oblivion singer, 33, has long since had ties to science fiction, with her long-term boyfriend Elon Musk launching his own SpaceX programme.
The company aims to launch an all-civilian trip into orbit this year, and eventually wants to land on Mars.
And Grimes herself wouldn’t object to a bit of extraterrestrial action.
Real name Claire Elise Boucher, her unique style has often been described as “out of this world”.
She admitted in an interview with The Face that she’d be up for uploading her consciousness into a “humanoid vessel that can speak and move freely”, which she can then send to Mars.
The Canadian musician said she’d be up for relocating to Mars after she turns 50 to help establish a human colony there.
Grimes said she’s “ready to die with the red dirt of Mars beneath my feet” in an Instagram post earlier this month, where she was pictured posing outside her boyfriend’s Starbase facility in Texas.
She and tech billionaire Elon have been dating since 2018, when they debuted as a couple at the Met Gala.
They have one child together – the controversially named X Æ A-Xii (pronounced X-Ash-A-12), who is 11 months old.