Yes, cats can be depressed. However, what may seem to be depression in cats could usually be narrowed down to your cat being physically ill. You should take note of the following before seeking help for your cat.
Depression in cats could come from cats being stressed or tense when moving into a new environment. This breaks up their normal routine. Cats do not like surprises and this may contribute to the depressive episode the cat is facing. De-stress your cat by providing areas like hiding areas for your cat to go to. Be patient and give it time to adjust to its new surroundings. Cats like being in control of their environment.
Having other cats around when a cat is new to the household could cause stress for the cat. You can assign a space for the new cat exclusively and give it time to adjust.
Eliminating stressors is one thing. Ailments such as terminal illnesses or medical conditions that cause physical pain would cause cats to seek comfort or withdraw. Should you notice your cat display such symptoms, a visit to the veterinarian to seek help would be recommended.