STD cases among millennials are up nationwide — and are spiking in the five boroughs.

Reports of syphilis have soared by 29 percent in the latest fiscal year to 1,700 — and people under age 34 account for most of them, according to city data.

Health Department figures also showed an increase of 6.5 percent in reported cases of gonorrhea (to 8,514) and of 6.2 percent in cases of chlamydia (to 32,604) when comparing Jan. 1 to June 30, 2016, with the same six-month period in 2015.

The bulk of new cases of sexually transmitted diseases were in the 15 to 34 age range.

“It’s certainly been well documented that among adolescents and young adults, those who are sexually active tend to have multiple serial relationships or concurrent relationships. Within the context of unprotected sex, that could amplify the spread of disease,” said Dr. Susan Blank, a deputy commissioner at the city Health Department.

“I think what it underscores is whether there is a decrease [in condom use] or not, the fact is that we need to encourage condom use.”

The Post reported Thursday that condom use nationally is dropping among young folks.

A survey of city high school students found in 2013, the most recent year available, that 67.8 percent reported using a condom the last time they had sex — down from 76.4 percent in 2003.

Officials noted that each disease affects different population groups at different rates: Chlamydia is more common among younger people and women, while syphilis is more prevalent among people in their 30s and gay men.

“The groups that are most affected in New York City and nationally really are young adults, men who have sex with men, and persons of color,” said Blank.

She said the city is doing its part — including by giving away nearly 36 million condoms in fiscal 2016.

National figures for calendar year 2015 show a 19 percent increase in reported cases of syphilis, a 12.8 percent increase in gonorrhea cases and a 5.9 percent increase in chlamydia cases compared to 2014, according to the Centers for Disease Control and Prevention.

Alarmingly, STDs are also becoming more drug-resistant: