The actor has been in the U.K. capital this week (beg29Jun09) to promote new movie Public Enemies, and took time out from his busy schedule to visit Great Ormond Street Hospital to cheer up young patients.
Dressed as his Pirates Of The Caribbean character Captain Jack Sparrow, he visited several wards - including the one his daughter Lily-Rose stayed in when she was treated for kidney failure in 2007.
A source tells WENN, "Johnny was so friendly with all the children on the ward and they absolutely loved his pirate outfit.
"He spoke to lots of the youngsters and staff - and made their day by posing for pictures with them all."
The generous star has previously donated $1.5 million (£1 million) to the hospital to thank them for looking after his 10-year-old daughter during her health scare.