A four-year-old cat called Biffy had to be put down after returning to his home in Culloden, near Inverness, showing signs of being poisoned on May 1.
Three cats living on the same street died in March 2012, while three cats from a nearby property have died between April last year and January this year.
The Scottish SPCA said all seven pets died from antifreeze poisoning but inspectors are unable to say whether or not the deaths have been accidental.
Andy Brown, senior inspector at the charity, said: "We are extremely concerned by the number of cats dying from antifreeze poisoning in such a small area and short space of time.
"Yet another beloved pet has lost its life and Biffy's owner is understandably devastated, as are all the other families whose cats have died under such awful circumstances.
"One man from Hazel Avenue has now lost three cats and has even considered moving home because of this.
"Unfortunately, we have not received any information about the source of the poison so we are unable to say whether someone has deliberately caused these cats harm or the deaths have been accidental.
"Either way, it is clear there is a source somewhere in the area close to Walker Crescent and Hazel Avenue."
He said antifreeze, or ethylene glycol, is usually colourless and odourless, with a sweet taste which appeals to cats. It causes "tremendous suffering" if ingested, with symptoms including vomiting, lethargy and head shaking.
Mr Brown added: "If anyone has antifreeze in their garage or shed we would urge them to ensure roaming cats do not have access to this.
"We understand it is difficult to keep outdoor cats inside but we are asking owners to be aware of the dangers.
"Anyone who believes their cat may have swallowed poison should seek veterinary attention without delay."