The 'One That Got Away' singer had the design - a smiling peppermint sweet with big eyelashes - done after a concert in Los Angeles last week, while husband Russell Brand also had a design on the back of his arm; the logo of his favourite soccer team, West Ham United.
Following his work on the 27-year-old pop beauty, Katy then turned the gun on him, giving him the same design on his leg.
She has been working up to the body art for some time, noting on twitter last week: "I'm getting MY tattoo TOMORROW! #tourtat (sic)"
Katy is a big fan of tattoos, and previously revealed she once came close to having Freddie Mercury's likeness recreated on her back once.
She said: "Freddie Mercury was - and remains - my biggest influence.
"The combination of his sarcastic approach to writing lyrics and his 'I don't give a f**k' attitude really inspired my music.
"I nearly got a huge tattoo of Freddie Mercury's face on my back a few years ago. It's probably for the best that I didn't get it done, but I still want to do it."