The stargazing event lasts from 6.30pm to 9.30pm and will also include a presentation on the wonders of the world and the universe.
Swansea Council has arranged the event.
Visitors will also have the chance to take a closer look at stars and other planets through a number of telescopes. Question and answer session will be arranged to satisfy the curious.
Lively indoor lectures will be held if the weather means stargazing is impossible.
Cllr Nick Bradley, Swansea Council’s Cabinet Member for Regeneration, said: “Astronomy is a fascinating subject and this event will allow people to find out more about the night sky and quiz experts. The night sky can seem a jamble of stars at first but, with a little bit of know-how and practice, people can often soon pick up the wonders of astronomy. Oystermouth Castle is also a terrific venue for the event because of its history and the atmosphere it evokes.
“The attraction offers a first-class visitor experience for history fans but it’s important we continue to use it as a location for other events as well. The stargazing event follows on from the success of several outdoor theatre performances at the attraction over recent years.”
Visitors are asked to wear appropriate footwear and warm, waterproof clothing because it may be very cold and the castle grounds are steep and uneven.
Tickets cost £1.50 and are available from either www.ticketsource.co.uk or the Swansea Tourist Information Office on 01792 361302.
Free hot and cold drinks will be provided on the night.