The facility will offer medication-assisted treatment and counseling to adolescents and adults, officials said.
Mountainside, which operates drug and alcohol addiction treatment centers, has opened an outpatient center in Huntington.
The 5,000-square-foot facility is located at 131 E. Main St.
The organization also operates facilities in Manhattan, Chappaqua, Wilton, Connecticut, and at its main campus, in Canaan, Connecticut.
Mountainside Huntington will offer individual, group and couples counseling, psychiatric services, medication-assisted treatment, adolescent services and health and wellness education.
The office, in leased space, will employ five people to start, a spokewoman said. Mountainside employs about 300 people total.
Fatal opioid overdoses in Nassau and Suffolk counties decreased in 2018 after alarming spikes at the height of an epidemic that has claimed almost 3,700 lives on Long Island since 2010.
Fatal overdoses fell to 483 in 2018, from a high of 614 in 2017, according to data provided by the police departments and medical examiners in Nassau and Suffolk. The figures are estimates because in some cases toxicology reports can take years to complete.
In the past nine years roughly 3,685 people fatally overdosed on opioids on Long Island.