The animated film, which is also narrated by Fry, shows Father Christmas developing dementia and being unable to deliver presents. However, the film also shows that dementia, which is caused by physical diseases like Alzheimer's, can be beaten through research.
Fry's blog, in which the broadcaster highlights the research and development work carried out by the charity's scientists, is available to read on the Alzheimer's Research UK website.
The original cartoon was produced by Aardman, the studio behind Wallace and Gromit, and devised by Freuds; though PR for this year's campaign is being handled in-house, the charity said.
According to Alzheimer's Research UK, 200,000 more people have developed dementia in the UK since the campaign launched last year, making it the country's leading cause of death.
Hilary Evans, chief executive at Alzheimer's Research UK, said: "The fact that dementia has become the UK's leading cause of death this month is a stark reminder why the messages in Santa Forgot are just as important this Christmas as they were last."
Last year's campaign film garnered almost 200,000 views on the charity's YouTube channel, and was voted the best Christmas campaign from November 2016, according to a poll of its readers by PRWeek UK.