Noir, a fairly small, all-black, two-year-old female cat, escaped from its new home in Donnington on December 28, 2017, having only been there a few hours.
The Cat and Rabbit Rescue Centre in Sidlesham which rehomed the animal says it does not know the area and may have travelled some distance by now.
Volunteer Pamela Fitzsimons said: “We have had teams and teams out looking for her and dropped 800 leaflets but nothing, so now we need the help of Chichester residents.
“Have you seen her in your garden or walking past? It’s been a long time now and we are getting desperate to find her.”
Noir is chipped and could still be wearing a yellow collar.
The rescue centre said the cat is friendly but will probably be very scared as it has never been an outdoor cat.
Anyone with information is asked to call the centre, which is offering a reward, on 01243 641409.