City officials said preparations had been underway for weeks, with police, transit officials and emergency crews coordinating safety and transportation plans as large crowds gather in the neighborhood.
The event is a major annual tradition in Boston, a city with deep Irish roots, and often coincides with celebrations of Evacuation Day, which commemorates the departure of British troops from Boston in 1776.
Officials encouraged people attending the parade to plan ahead for road closures and crowded public transit as the neighborhood fills with spectators throughout the day.
Last year’s parade drew tens of thousands of attendees and featured dozens of marching groups.
This year’s event was expected to begin late Sunday morning and continue into the afternoon.