A large team of volunteers, with support from local businesses and individuals, have over the past three years developed the trail into a fenced route through farmland, which gradually rises to Woodside.
It passes heritage trees and has picnic spots and views of the Tararuas along the way. The metal surface is suitable for walking and bikes.
The opening ceremony will be held at the Woodside end of the trail, by the Woodside railway station, at 11.15am.
Participants are invited to bring along a picnic and refreshments will be available.
The event will include prizes for best costumes and dogs are welcome on a lead.
Chairman of Greytown Trails Trust Gerad Taylor said it was great to have the trail finally completed.
"It's been four years since I had the initial idea, and there's been a few obstacles along the way, but it's fantastic to see it completed.
"The enthusiasm we've had from the community is amazing," Mr Taylor said.
He said the Greytown trail was one of the New Zealand's top trails.