The body of a dead dog was found in a zipped-up rucksack with its legs bound together with cable ties.
The male Staffordshire bull terrier was discovered by a tree surgeon in woodland near Dibden Purlieu, Hampshire, on Wednesday.
It had also been wrapped in a duvet cover which was sealed with gaffer tape, the animal charity said.
The dog was not microchipped so its owners have not been traced. The RSPCA is appealing for information.
'Scabbing and bruising'
Inspector Penny Baker said the animal was "very deliberately placed deep in undergrowth".
"He was in normal bodily condition with some scabbing around his groin area and bruising inside his mouth but it is not clear how he died," she said.
"I'd be really interested to know where this dog has come from and how he ended up buried in a bag with his legs bound together."