The US singer revealed the news in the music video for her latest single, Never Worn White.
The romantic ballad appears to have been inspired by her relationship with the 43-year-old British actor, who she has been dating since 2016.
In the clip, Katy sings: “See us in 60 years with a full family tree.”
The video ends with a side-on shot of the Firework singer cradling a baby bump.
Confirming the news on Instagram, the 35-year-old star said: “Let’s just say it’s gonna be a jam packed summer.”
She also joked about no longer having to hide her bump, tweeting: “Omg so glad I don’t have to suck it in anymore or carry around a big purse lol.”
In a live video, Katy also revealed she is ready to release a new album, saying: “Let’s call it a double whammy.”
The singer added: “I am excited, we’re excited, and happy. It’s probably the longest secret I’ve ever had to keep.”
Katy said she wrote Never Worn White in Santa Barbara and the intro of the track is from Felix Mendelssohn’s Wedding March.
“It’s a super vulnerable song for me to put out there,” she said. “It’s very honest, it’s about surrendering to love and all that you go through when you’re about to commit to someone.”
Addressing her previous marriage to Russell Brand, Katy told fans the title of her new song is accurate as she wore a dove grey dress for the ceremony.
“The truth is I have never worn white,” she said. “Because the time before this, 10 years ago, I wore dove grey. So let’s just say it’ll be my first time wearing white.”
Katy got engaged to the Pirates Of The Caribbean actor on Valentine’s Day last year.