Captioned 'full bloom', the image showed the couple against a backdrop of red balloons, with Perry showing off a big red ring - seemingly on her engagement finger.
Bloom then posted the same photo, writing: "Lifetimes."
"Are they engaged?" one fan asked.
"Congratulations Katy & @orlandobloom! I'm so happy for you both!!" another wrote.
"I'm crying of joy! You deserve it!" a third said.
I HOPE YOU'LL BE SO HAPPY TOGETHER ILY :heartpulse::gift_heart::two_hearts::revolving_hearts::purple_heart:❣️:two_hearts::revolving_hearts::heart::sparkling_heart: pic.twitter.com/25VHaVr7QY
- ♡ (@diasonrant) February 15, 2019
"We (I) need the engagement story," someone else begged.
According to the Mirror, the singer's mum, Mary Hudson, also took to Facebook to congratulate the happy couple, sharing a photo of the ring and writing: "Look who got engaged on Valentine's Day!!"
Singer Perry and Lord of the Rings actor Bloom first started dating in January 2016, before confirming their split in February 2017. However, they reunited last April, and have remained together since.
Bloom has a son, Flynn, with ex-wife Miranda Kerr, while Perry used to be married to English comedian Russell Brand - separating in December 2011 after 14 months of marriage.
Speaking about her short marriage to Russell Brand, she once told W Magazine: "I really loved it. It was a new, fun, exciting journey.
"I like how I am now, and I think all things happen for a reason."
More recently, however, she's discussed the prospect of marriage not necessarily being for her.
In an interview with Paper magazine, published just a few days ago, she said: "I'm very pragmatic and logistical and I am less fantastical about things... I mean, I was married when I was 25...
"I'm 34. It was almost 10 years ago. I was like, 'One person for the rest of my life', and I'm not so sure that that idea is for me. I'm just such a different person than I was."
But she also explained what can be gained from being with someone, and the benefits of being in a relationship.
"It's easy to be single and not to be called out on your stuff, living your awesome single life thinking you're the greatest, and the best, and doing anything you want," Perry said.
"But then when you get in a partnership you guys are there to teach each other lessons."