Sir Bruce Forsyth has called for a change in the law on assisted dying so that people can choose to die with "a bit of dignity left".
He said watching his first wife Penny Calvert spend her last years in a care home with dementia had convinced him people should not be "left to suffer".
Sir Bruce visited regularly to help with her care until she died last year.
He told the Mail on Sunday it could often be "more cruel to do nothing" than to let someone die.
"It's a terrible illness, awful to see and awful for everyone close to the person suffering," he said of his first wife's dementia.
"It's a very difficult situation. She didn't remember me. It's just so awful to have your mind and your memories taken away from you."
Sir Bruce, 87, continued: "It bothers me an awful lot that people are just left to suffer. If I had Alzheimer's or dementia I would do something about it.
"The law should be changed and if people want to die with a bit of dignity left they should be able to do so.
"If it is what the person wants and it can be proved they are living a life of suffering it can be more cruel to do nothing. I'd like that right for myself."
The Alzheimer's Society said: "Everyone experiences dementia differently and it can be extremely difficult and painful to find yourself losing your memories and abilities. It is important that people with dementia know there is information and support out there so they don't have to struggle.
"The Alzheimer's Society is not calling for a change in the law on assisted dying or euthanasia. We believe that people with dementia are entitled to good end of the life care. Improving staff training and awareness is vital to make people's final days as comfortable as possible.
"This must include the maintenence of dignity of each person and their loved ones."
Sir Bruce married Ms Calvert in 1953 and they had three children before they split up 20 years later. However the pair remained on good terms and he helped nurse her through her illness after she went in to a home in 2008.
The veteran presenter, who quit as the host of Strictly Come Dancing last year, has been married to third wife Wilnelia Merced since 1983.
Other names who have spoken out in support of assisted dying include late author Terry Pratchett, Sir Richard Branson and Professor Stephen Hawking.
Labour peer Lord Falconer has tabled a bill asking that patients with less than six months to live be allowed to take their own life with the help of a doctor.
However Judge Baroness Butler-Sloss is against the bill, which she says would "cross a line" towards euthanasia.
Alistair Thompson from Care Not Killing told the BBC: "Clearly comments like these are disappointing, but what we need to do is look at the level of care.
"There are cases where people have not received the right level of care and that is what we should be looking at or assessing - rather than making such a huge step in the law and saying, 'you are not entitled to the same levels of protection as the young and able bodied'. Rather than putting people at risk."
He added: "When you look at the detailed polling on this issue, there is a huge amount of concern among the general population about how people might feel pressured into ending their lives - because they may feel like a care burden or a financial burden.
"It sends a very bad message."