Deadpool star Ryan Reynolds has penned a touching farewell message for a 13-year-old fan who died of cancer this week.

"Connor was 13. But this kid... He was smart. He was funny. And not just funny 'for a kid' - or funny 'for a person battling something awful'. He was unqualified funny. He had that... thing," Reynolds wrote in a Facebook post on Thursday (April 28).

Canada-born Reynolds, 39, met Connor McGrath through the Make-a-Wish Foundation. The international foundation grants wishes to children who have life-threatening illnesses.

He said that Connor, who had high-risk acute lymphoblastic leukaemia, was a huge fan of Deadpool.

Connor was diagnosed in 2013 and relapsed last May, the Edmonton Journal reported.

"I'm grateful I got to orbit Connor's world for a brief time. Grateful for the pages and pages of hilarious texts between us. Grateful to his parents for allowing Connor to spend time with a foul-mouthed child in the body of a 39-year-old," Reynolds wrote.

Because of his wish, Connor was the first person to see the Deadpool movie. Reynolds travelled to Edmonton where Connor lived to surprise him with a rough cut of the film, he said.

They became friends and Reynolds called Connor "Bubba". Reynolds also went to Stollery Children's Hospital to hang out with Connor earlier this year, the Canadian report said.

"We met because he loved Deadpool. In a certain sense he WAS Deadpool. Or, at least everything Deadpool aspires to be; balancing pain, fearlessness, love and a filthy (filthy!) sense of humour in one body. I wish he could've stuck around a lot longer," said Reynolds.

Connor's family was trying to raise US$800,000 (S$1,075,440) for an experimental treatment at the Children's Hospital of Philadelphia, said the Edmonton Journal. An online campaign had raised US$35,522.

Reynolds said: "My deepest condolences and thanks to Kim, Gerald and the entire clan McGrath - who loved this little boy so very well. And thankfully, Connor knew it.

"See ya down the road, Bubba."