The black and white tabby males were found outside a business in Redhill Drive in Bournemouth on Monday morning and rushed to the RSPCA clinic.
Their condition was so bad that vets were left with no choice but to put them down.
One could not use his legs at all and could only roll and the second was able to move by dragging himself.
Both also had what appeared to be a constant tremor and were clearly in pain and distress.
RSPCA Inspector Jo Story said: “They were about three months old so someone has been potentially trying to keep them alive for that amount of time despite their obvious condition.
“These animals were suffering. By the time they were picked up there was no way they could have had any sort of normal life. It was very distressing for the staff here.”
Insp Story added: “Our concern is that someone decided to leave these kittens in the state they were and not seek the help of a vet. Even if they didn’t have any money, the PDSA or RSPCA could have helped.
“Another inspector said it was one of the worst things that anyone’s seen for quite some time.
It was very distressing because the deformities were so obvious.” Anyone with information is asked to call the RSPCA cruelty line on 0300 1234999.