The building is fully let to Citibank and houses the international bank’s support services operations.
Made up of three connected accommodation blocks with a central courtyard, work on the building by owners Titanic Quarter was finished in 2009.
CBRE has been appointed by Titanic Quarter to sell the property and said there was strong demand for Grade A offices.
A spokesman said: “The Belfast office sector is buoyant following a record year with occupier take-up estimated in excess of 800,000 sq ft.
“Rental growth has also been strong with headline rents in the city expected to achieve £22 per square foot by year-end.
“A lack of suitably configured Grade A space coupled with continued high levels of occupier demand is encouraging both developers and investors to focus on this sector.”
Commercial director at Titanic Quarter, James Eyre, said: “Titanic Quarter continues to grow with over 1.5 million sq ft of space completed and occupied with over 18,000 people living, working and visiting daily.
“Titanic Quarter is established as a centre for premium Grade A offices and business innovation, a world class tourist destination, an education and media campus and a prime residential community.
“The Gateway Offices therefore provides an excellent investment opportunity.”