The couple - who have 14-month-old daughter Daisy together - donned medical-themed costumes for the spooky holiday on Sunday (31Oct21) in a nod to the ongoing coronavirus pandemic and the vaccination programme.
Orlando shared a photo of the pair in costume on Instagram and, in a play on the lyrics of his 37-year-old fiancee's 2008 single "I Kissed a Girl", he captioned the post, "I vaxed a girl and I liked it."
Katy shared the same photo, as well as a selection of others, including one of them posing on the steps of a clinic, with the simple caption, "Stay safe guys (sic)"
The "Lord of the Rings" actor - who also has son Flynn, 10, with ex-wife Miranda Kerr - wore white Crocs and a white medical coat over blue scrubs, while Katy adopted the same colour scheme for her giant syringe outfit.
Before their Halloween celebrations, the couple marked the "Roar" hitmaker's 37th birthday on 25 October with a few days in Cabo San Lucas, Mexico, along with a group of friends.
Sharing a selection of photos from the vacation on Instagram over the weekend, Katy wrote, "When you're 37 hangovers last 5-7 business days (party emojis). (sic)"
The 44-year-old actor also shared a photo from Katy's birthday dinner, in which she wore a "Birthday Girl" tiara as she prepared to blow out the candle on her cake.
He captioned his post, "We do life we do love and it's fun. I'll celebrate you today and everyday. (birthday emojis) I love you. (heart emojis) (sic)"
Katy shared a gushing birthday post of her own when the "Pirates of the Caribbean" star turned 44 in January.
She wrote at the time, "Happiest 44th to my love, brilliant father of my Dove & a shimmering mirror that sees and reflects to me what I can't yet see…So glad my moon found it's sun. love you whole world doe (heart emoji) .(sic)"