Collins, who has finished filming as Weatherfield's Stella Price, will play the role of Samantha on the BBC medical drama. Her first scenes will air in July.
Samantha will end up in hospital after being injured while trying to help rescue a competitive father, who has a heart attack after a running race with his son.
The character will also be a new love interest for paramedic Jeff, who treats her at the scene.
However, as the romance with Jeff blossoms, a secret held by Samantha threatens to turn both of their worlds upside down.
Speaking about the part, Collins said: "I got offered the guest role in Casualty very soon after finishing Corrie. I jumped at the chance of playing a character so different so quickly - it was exciting to be stepping into something new.
"Playing Stella was great fun but it was thrilling to have a challenge of an entirely different role. Londoner Samantha is nothing like Stella - I dyed my hair to leave Stella behind which fitted in well with the role.
"I'm pleased to be involved in another hit drama TV show so soon, even if it's just for a short while. Since leaving Corrie it's been great to get my teeth into different characters and I'm looking forward to new opportunities which have been coming my way."
Casualty's executive producer Oliver Kent added: "We can't wait for everyone to meet Samantha and see where her story goes over the two months she's on screen. Michelle has been absolutely brilliant and her arrival is set to have a huge impact on one of the show's most loved characters."
It was announced last year that Collins would be leaving Coronation Street. Her final scenes will be aired next week as Stella leaves Weatherfield for a new life in New York.
Digital Spy Soap Scoop video - hit play below to find out who Leanne shares her domestic violence secret with in Coronation Street, why Aunt Babe finds herself under fire in EastEnders, how Carly returns in Emmerdale and why a Hollyoaks killer secret comes under threat.