The RSPCA warned pet owners after a concerned member of the public from Sevenoaks Road, Ely, contacted them.
RSPCA acting chief inspector Selina Chan said: "The caller stated four cats died in their garden and knows of another three that have died. This must have been very traumatic to witness."
Poisoning a cat can bring six months in prison or a Â£20,000 fine.
Signs that a cat has been poisoned include lack of appetite, vomiting, diarrhoea and difficulty breathing.
The RSPCA is investigating the incidents.
Seven cats have died from suspected poisoning in a Cardiff suburb in two months.