Kevin Johnson left £364,568.68 to the animal charity in a final touching tribute to his beloved Alsatian, Chelsea.
The 59-year-old adopted Chelsea, who had been badly mistreated, seven years ago and the two had been inseparable ever since.
'She was the apple of his eye and his biggest friend'
However the 17-year-old dog suffered from an unexpected stroke, and had to be put down.
Just two months later Mr Johnson suffered a fatal heart attack, and died in his sleep.
Speaking about his devastation, Mr Johnson’s aunt, Myfanwy Gillham, 90, of Enfield, said:
‘Kevin was totally devastated at losing Chelsea – she was the apple of his eye and his biggest friend.
‘He just lost heart after she went.’
Describing the moment the nursery man adopted Chelsea, Gilham revealed:
‘We went to Southridge together one day on the off chance of finding one [a dog] and immediately spotted Chelsea.
‘They bonded straight away from their very first meeting as soon as he set eyes on her.
‘Chelsea was always with him, playing all day at the garden nursery. He never went anywhere without her.
‘And then, one terrible morning, he came downstairs and found Chelsea collapsed.
'He never went anywhere without her'
‘Kevin couldn’t get over it and started letting his business go.
‘Two months later he had a heart attack and was found dead on his bed.’
Johnson left all his savings to the RSPCA as well as all the cash from the sale of his cottage.
Terry Pavey, chairman of the Friends, said:
‘Words cannot express enough our gratitude to Kevin for leaving us this staggering inheritance.
‘It will do so much good, notably safeguarding our fostering scheme for years to come.’
A new dog agility and play area at Southridge will now be dedicated to the memory of Kevin and Chelsea and a special plaque is to be put up in their memory.