The Royle Family star dug deep to ensure 15 more apartments could be built at Ronald McDonald House in Liverpool. Despite being one of the biggest of its kind in Europe, the accommodation is full every night with families of patients at Alder Hey Children's Hospital.
The comic actor said: "The house provides a vital lifeline to parents and families who come to hospital with their critically ill children and these new apartments will be able to give them some much needed support during an incredibly difficult time.
"I'm looking forward to seeing them completed in June 2011."
Jan Thomas, the charity's house director, said: "The apartments will provide our families with privacy, normality, and most importantly their own front door.
"This is a really exciting time for us and we are thrilled that Ricky has joined us today to take part in our celebration.
"I would like to thank him for his generous donation along with all those families and other supporters who have all played their part in making this fantastic new development happen."
Tomlinson's cheque will pay for the majority of the 1.5m development.
Charity bosses were stunned when they realised the size of the donation.
Dozens of children and their families gathered at Ronald McDonald House, situated at Alder Hey, to watch Tomlinson place a time capsule in the extension's foundations.
It contained letters, cards and poems from families, along with a child's footprint, photographs and a copy of the Liverpool Echo newspaper.
The house currently provides 69 bedrooms but is unable to meet the demands of a lengthy waiting list. The extension will provide accommodation for 84 families.