The actor portrayed charge nurse Martin 'Ash' Ashford on the BBC One drama for eight years until 1996. Robinson's last appearance on the show saw Ash as best man at the wedding of Charlie Fairhead (played by Derek Thompson) and Baz.
Ash's return to the emergency department will reveal he has retrained as a doctor. The upcoming episode If Not For You will see him attend a motorbike crash on the way into work. He quickly makes an impression on his colleagues, and clinical lead Zoe (Sunetra Sarker) is forced to leave him in charge of the department when she comes down with flu.
The popular character's return follows the departure of Nick Jordan, played by actor Michael French.
Executive producer Johnathan Young commented: "We're thrilled to have Patrick back. Ash was a hugely popular character and reuniting him with Charlie will reignite a friendship that we hope will delight viewers old and new.
"With many years of nursing under his belt plus a stint working as a doctor in America, Ash brings a unique perspective to the ED that will challenge Zoe but charm the rest of the team."
Robinson added: "It's fantastic to be back! Recreating a character that has moved on and changed so much is incredibly challenging and this feels like a totally new job. But to be back working with Derek Thompson, I feel like I've come home."
Casualty: If Not For You will be shown on BBC One on Saturday, February 9. Get the Inside Soap magazine on your iPhone or iPad