The alert has been issued by the UK Health Security Agency and covers the entire East of England.
It will be in place from 9pm on Thursday to 8am on Sunday.
Temperatures are set to hit 28C in parts of Norfolk by Friday.
Under the Met Office’s Weather-Health alerting system, a yellow alert means there could be an increased use of healthcare services by vulnerable people.
It may lead to an increase in risk to health for individuals aged over 65 or those with pre-existing health conditions, including respiratory and cardiovascular diseases.
It comes after the Met Office issued a yellow thunderstorm warning for Norfolk.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, said: “Our findings show that even moderate heat can result in serious health outcomes, especially for older adults, and it is therefore important that everyone takes sensible precautions while enjoying the sun.
“The forecasted high temperatures are expected to be short-lived but could primarily impact those over the age of 65 or those with pre-existing health conditions.
“If you have friends, family or neighbours who are more vulnerable, it is important to check in on them.”