The south Belfast offices of Power NI parent company Viridian have gone of the market for £475,000.
Agents Lambert Smith Hampton have now been appointed to sell the two-storey office building on Newforge Lane - one of the most expensive areas in Belfast for residential properties.
The 3,456 sq ft property is let out to Viridian at a 'passing' rent of just over £34,500.
A spokesman for LSH said: "We expect a high level of interest for what is an attractive lot size and a secure investment opportunity.
"Viridian are an excellent tenant whom have been in situ in New Forge for many years.
"The tenant had recently taken a new lease and completed a refurbishment of the property indicating their long term commitment to the location."
The name of the private investor who is selling the site has not been revealed.
However, Viridian's tenancy is not affected by the sale.
Power NI is Northern Ireland's biggest electricity supplier. Viridian formerly owned the networks side of the business, before a sale to ESB in the Republic in 2010.
Viridian also owns Energia, a major supplier in the Republic.
Viridian was bought over by private equity firm I Squared Capital in a deal reported to be worth €1bn (£0.78bn) around two years ago. It was sold by Bahrain's Arcapita, which had owned the company for around 10 years.