The actress - best known for her role as Lisa Hunter in the Channel 4 soap - will play Tamzin alongside second new recruit Omar, who will be played by Sex and the City 2's Dhafer L'Abidine.
With Tamzin tipped to cause a stir among males at the hospital, Atkinson said of her new role: "I am absolutely thrilled to be joining the show and really looking forward to getting to know all the cast and crew.
"I used to watch it as a child so to be involved is fantastic."
Pictured: Dhafer L'Abidine and Gemma Atkinson.
Atkinson previously claimed that she had turned down an offer to return to Hollyoaks because she did not want to "get stuck in that rut again" while focusing on movie work.
Casualty producer Oliver Kent said: "Tamzin is going to bring a touch of glamour to the paramedic team and set some hearts racing in the emergency department while Omar's melting roadside manner is going to raise the temperatures of Holby's female patients."
Atkinson and L'Abidine begin filming this month and will make their on-screen debut in the BBC show from September.
Casualty entered the Guinness World Records last year after being named the longest-running primetime medical drama in TV history.
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