The animal welfare charity was alerted after walkers spotted the sheep stranded on the ledge at the Trefor Sea Stacks on the Lleyn Peninsula, an area popular with Shropshire holidaymakers.
No grazing was available on the cliff edge for the lamb, while she was surrounded by nesting sea birds who would peck her every time she moved.
The terrifyed lamb faced a 20-metre drop, onto rocks or the sea and was suffering in the heat.
RSPCA inspector Mike Pugh got grass to the stranded sheep, before notifying the farmer who provided hay and water.
A five-strong team rescued the lamb, putting it into a bag and lifting it to safety, before returning it to her near-by mother.
Inspector Pugh said: "This poor lamb was horribly stranded on a cliff ledge and unable to get herself back to safety. The poor young sheep was being pecked by nesting sea birds every time she moved.
"We were later able to reach the lamb, by abseiling to her and pulling her back using a pulley system to safe land. I waited at the bottom in case she fell into the sea - but thankfully it was a highly successful rescue.
"Fortunately, despite her ordeal, the young sheep had no injuries and we were able to reunite this lucky lamb with her mother.
"What was really nice is that - after we rescued the lamb and were packing up our equipment - she popped back over to see us. Perhaps she was saying thank you!"
Farmer Arwyn Owen, who owns the lamb, said he was very grateful to the RSPCA for the rescue.
The lamb may have fallen onto the cliff ledge while escaping a dog and the RSPCA renewed its calls for dog walkers to keep their pets on leads near livestock.