Japan's weather officials say temperatures will be higher than average across the country from May through July.
The Meteorological Agency announced a three-month forecast on Tuesday.
It says warm air will tend to blanket the country during the period as a Pacific high is likely to extend more westward than usual.
The agency expects temperatures at around average or higher in northern Japan, and higher in eastern and western Japan, as well as the Okinawa and Amami regions.
The agency says, depending on other conditions, the mercury may top 30 degrees Celsius even in May and June.
The agency is advising residents to take precautions against heatstroke.
Agency official Tanaka Shotaro said if temperatures surge when the body has not become used to summer heat, that increases the danger of heatstroke. He warned people to be careful even if a heatstroke alert is not issued.