The Heritage Lottery Fund contributed Â£3m to the Â£5m conversion of Itford Farm, near Beddingham, Lewes in the South Downs National Park.
The project has taken 13 years, with the Youth Hostel Association (YHA) taking over as developer along the way.
The charity said the hostel would open to overnight guests from 5 April.
An Easter egg hunt and other family activities were planned for Sunday's open day.
"YHA South Downs has been a long time coming," said manager Dale Richards.
"Giving local people the chance to have a sneak peak and join us for a fun day is our way of saying thank you for their patience."
Area manager Lisa Venter said the YHA stepped in as developer during the final year after the project stalled.
The Greenbank Trust, Active Lewes, East Sussex County Council are among other organisations which helped fund the 68-bed hostel, which was a working farm until the 1980s.
YHA South Downs will also have a cafe and visitor centre for walkers and school parties.
"It's vital that visitors to the South Downs feel connected to the National Park's landscapes, wildlife and communities as that's the best way to encourage people to take care of them," said Trevor Beattie, chief executive of South Downs National Park Authority.
"The new youth hostel will be a fantastic gateway into the South Downs National Park."
An open day has been held at a converted 13th Century farm to allow local people a preview before it opens as a youth hostel.