The bull terrier type dog was found straying in Newill Close in Bradford on Thursday, March 28.
He had been starved, his ribs were clearly visible and he weighed only 21 kilograms.
A member of the public found him and took him in and contacted the RSPCA.
The dog, who has now been nicknamed Stanley, is currently being cared for by the dog warden.
RSPCA Inspector Emma Ellis said: “Stanley is extremely skinny and has clearly been starved. He weighs just 21kg.
“He isn’t microchipped and we’ve made enquiries in the local area but no one seems to recognise him. It’s not clear whether he’s straying from somewhere nearby or whether he’s been deliberately abandoned.
“He’s very skinny but, despite this, he’s super friendly and his tail hasn’t stopped wagging!
“He loves people and gets on well with others dogs so we suspect he’s come from a home environment.”
To rehome Stanley and other dogs like him, visit the RSPCA website.