In a study of more than 70,000 women, optimists were less likely to get fatal cancer, heart disease, lung conditions and stroke in their retirement years.
Although some of the association is explained by healthier life choices and behaviours, experts believe a positive mental attitude is powerful in itself.
And even if you lack a natural sunny disposition, optimism can be learned.
Encouraging people to imagine a bright future could be a good medicine to boost public health, say the authors of the study in the American Journal of Epidemiology.
The researchers looked at health data collected from a large US study of working and retired female nurses.
The participants had been asked to rate how optimistic they perceived themselves to be on a scale of zero to 24.
The healthy women, who ranged in age from 58 to 83, were monitored over the next eight years and any disease-related deaths were recorded. There were 4,566 deaths overall.
Higher optimism was linked with lower death risk, even after controlling for other factors such as whether the woman was married, came from a richer or poorer family background, or had a history of diabetes, high blood pressure, high cholesterol or depression.
A significantly lowered risk was seen for deaths from a number of causes, including heart disease, stroke, cancer, respiratory disease and infection over the course of the study period.
The researchers believe optimism may have a biological effect on the body as well as a psychological one, although they didn't look at this.
Other studies have linked a positive outlook to lower inflammation, for example, and better heart health.
Join the conversation - find us on Facebook.
Investigator Dr Eric Kim, from the Harvard TH Chan School of Public Health, said there were several strategies people might want to try for an optimism boost:
Or you could try a course of mindfulness or some sessions of talking therapy.
He said: "Twin studies suggest up to 25% of optimism might be genetic or inherited which would mean up to 75% could be modifiable."

The Furcast group says the 90-minute podcast went out without its knowledge and it is "deeply sorry".
Two Texas stations were among those which broadcast the material, aimed at "furries"- people interested in animals that are given human traits.
Broadcasters have been advised to change passwords on the hardware many of them use.
Barix streaming boxes are popular with broadcasters and PA professionals.
Furcast said that multiple server requests for its content during the incident were in the name of "Barix Streaming Client" and that many of the individual boxes involved were visible on Shodan, a search engine for devices connected via the Internet of Things.
The BBC has contacted Barix for comment but the problem appears to be with security settings not being updated by the box owners.
"Someone is attacking Barix Boxes," wrote a member of the Alabama Broadcast Association.
"Several radio stations and at least one radio network have been compromised. The Barix receiver is pointed to an obscene podcast and its password changed so it can only be reset manually."
Furries are people who have a fascination with anthropomorphism and often dress in animal costumes.
The furry group Furcast describes itself as "an improv comedy-themed furry podcast with no censor" and denies that its main aim is to create sexual material.
"Our content is discovered by individuals who specifically seek what we produce, and they do not normally come into contact with it via public means," they wrote.
"We have no interest in being discovered by a mainstream audience."
Texas radio station KXAX found itself broadcasting Furcast's podcast on Tuesday.
"At about 9am we were notified that a programme was playing on the station that did not originate from this studio," the station wrote on Facebook.
"We found out that our equipment had been hacked and was broadcasting a podcast or a stream from an unknown source.
"We were able to eventually get the problem resolved. But still want to apologise to anyone who may have heard the programming."
KXAX general manager Jason Mclelland told Ars Technica there did not appear to have been a reason for the hack.
Another station affected, KIFT, said in a statement that it had only been able to regain control of its output when an engineer physically went to the site of the hacked remote transmitter.
"We are working with equipment manufacturers and auditing the security of our own systems to avoid any repeats of this incident," it said.
Women who look on the bright side of life cut their risk of many deadly diseases, according to researchers.
[NEXT_CONCEPT]
Several US radio stations played out an explicit podcast to listeners after an apparent hack.