From the pyramids and the Leaning Tower of Pisa, to the Sydney Opera House and South Africa's Table Mountain: some of the most recognised man-made and geographical attractions will join a host of British landmarks to "go green".
In the UK, the London Eye, HMS Belfast, Selfridges department store and Liverpool FC's Anfield stadium will all turn a four-leaf-clover shade of green as part of the celebrations.
Other landmarks around the world will adopt what the Irish tourist board calls "global greening", including the Burj tower in Dubai, Niagara Falls, and the entire Costa Blanca town of Moraira.
In Dublin, up to 8,000 visitors from around the world marched in a "people's parade". The IrishTaoiseach, Enda Kenny, meanwhile, joined a procession through New York. Mr Kenny and President Barack Obama are scheduled to meet at the White House on Tuesday for a traditional exchange of shamrocks.
The newly elected Pope Francis sent a message to the people of Ireland to mark the day, greeting Michael D Higgins, the Irish President, and commending the nation "to the powerful intercession of St Patrick".