Peel Castle will reopen to the public from Friday, Manx National Heritage (MNH) has said.
The castle is one of several seasonal historical sites that have remained closed during the coronavirus outbreak.
The Peel site was chosen for a trial opening because there are "no roofs" on the medieval structure, MNH director Edmund Southworth said.
Heritage attractions across the island were closed on 20 March to help contain the spread of the virus.
The island has had no new positive Covid-19 cases for three weeks, and there are currently no active cases either.
As part of the trial, Peel Castle will only open for reduced hours on four days a week.
A maximum of 50 people will be allowed in at any one time.
Visitors will be encouraged to "take a particular direction round" the castle to allow for social distancing, and audio guides will be replaced with paper ones.
The crypt at the site will remain closed.
Visitors will also not be able to use the toilets.
The trial will be followed by the partial reopening of three other sites before the end of July.
Initially only the garden areas at Rushen Abbey and the Grove Museum will be reopened, while climbing the Laxey Wheel will not be permitted because of the danger of people "bumping into one another", Mr Southworth said.
No timeframe has yet been set for the reopening of the "more complicated galleries" at the House of Manannan and Manx Museum.
Mr Southworth said: "In an ideal world we'd open as soon as we can, obviously, but that has to be balanced against doing it safely.
"I'm not putting a date on it at the moment."
Public access to unmanned ancient monuments in the Manx countryside has been maintained throughout the lockdown period.