At least 146,000 cases were recorded from January to July 20, a 98 per cent increase on the same period last year, according to the Health Department.
The epidemic has been declared so that officials can identify areas in need of emergency attention, the BBC reported.
"It is important that a national epidemic be declared in these areas to identify where a localized response is needed, and to enable the local government units to use their Quick Response Fund to address the epidemic situation," Health Secretary Francisco Duque said in a statement.
The Philippines declared an initial "national dengue alert" in July. The global incidence of dengue has grown dramatically in recent decades, according to the World Health Organization (WHO).
Fears over a dengue vaccine led to a big drop in immunization rates in the Philippines for preventable diseases, officials warned in 2018.
Concerns were triggered over Dengvaxia, the world's first vaccine against dengue, after 14 children died out of more than 800,000 inoculated in 2016-17, the report said.
Sanofi, the French developer of the vaccine, and local experts, however, said there was no evidence linking the 14 deaths to the drug.