The weak male adult mute swan was discovered hiding in bushes by a member of the public who immediately contacted the Scottish SPCA for help.
Staff at the Scottish animal welfare charity's national wildlife rescue centre at Fishcross, near Alloa, carefully removed all of the oil and the swan is now being nursed back to health.
Centre Manager Colin Seddon said: "The person who found the swan in the bushes was concerned by his refusal to move despite the close presence of several dogs, a clear indication that he was in need of assistance.
"Our animal rescue officer brought the swan in to us and we gently removed all traces of diesel from his skin, using the specialist oiled bird facilities we have here.
"From the pattern of coverage it would appear that the swan had dipped his head and neck into a diesel oil spill which had possibly gathered in reeds at the edge of the pond.
"It is not uncommon for waterfowl to come into our care in this state, often as a result of people irresponsibly disposing of oil in ponds and waterways.
"This is a very reckless and dangerous thing to do as diesel is an irritant which can cause extreme burning to the skin and, if ingested, can also do a great deal of damage internally.
"Birds can and do die from the injuries they sustain from this type of oil so we'd strongly urge people to dispose of this material in a responsible manner.
"Thankfully we were able to clean and treat the swan before it was too late, although he has lost a lot of neck feathers due to the burning effect of the oil and he was underweight on arrival, suggesting he may have swallowed some of the diesel too.
"We hope this swan will continue to improve and put on weight and that we'll be able to return him to the wild in a few weeks' time."
Anyone who discovers an injured or distressed animal should call the Scottish SPCA Animal Helpline on 03000 999 999.