The Belfast parade departed from city hall at noon and is making its way to Custom House Square for a free concert.
Parades and celebrations are also being held in Londonderry, Armagh, Newry, Enniskillen, Omagh and Downpatrick.
The largest parade in the Republic of Ireland is in Dublin.
There will be some disruption to traffic between 11:45 GMT and 13:30 GMT during the Belfast parade in the area of City Hall, Royal Avenue, Donegall Street, Waring Street, Victoria Street, Custom House Square.
Arterial routes remain open and alternative routes are available.
In Derry, thousands are expected to line the route of the carnival parade. This year's theme is "Land of the Little People".
Thousands of people are attending St Patrick's Day parades and festivities in towns and cities across Northern Ireland.