Staff at The Christie found the anonymous note on Sunday on a machine where the tray had been left full of paid-for snacks.
The donor said they hoped the "random act of kindness" would "brighten up someone's day".
Nurse Emma Widdowson said: "It made me feel so happy I was beaming all day."
"Who doesn't want choccie on a Sunday?" she added.
Health care assistant Lauren Hunstone said: "It just perked up everyone's day that someone would do something so lovely out of the goodness of their own heart, not wanting to take any sort of thanks."
The note said: "Please find a variety of snacks in the bottom of this machine that have been paid for.
"I simply wanted to perform some random act of kindness to try and brighten up someone's day.
"Hopefully there will be something in there for you to enjoy.
"Finally, if you can, try and spread the kindness."
The Christie was recently praised by health inspectors for providing "exceptional" care and being a "leader in its field".
It is the largest single-site cancer centre in Europe and treats more than 44,000 patients per year.
A note was left on a vending machine at a cancer hospital in Manchester asking people to help themselves to free treats.