The 190,000 ft² (approx. 17,651 m²) building is 94% let to tenants including Admiral Insurance, Tiger Tiger, the government, the Law Society and Hilton Hotels.
Admiral accounts for more than half the income but is due to vacate the building in 2.5 years in favor of a new headquarters.
Clive Bush, Managing Director - Operations at Topland Group, said: "We are delighted to have completed this purchase, which will provide us with a number of active asset management opportunities, which will be worked up by our in-house management team. Our ability to purchase with cash has again proved a decisive factor."
DTZ Senior Investment Director Nick Allan said: "This is the most significant and high-profile investment transaction within Cardiff this year and was achieved in very challenging market conditions for secondary office buildings.
"The buildings provide many opportunities in the future for redevelopment and/or major refurbishment. The buildings, views and location are clearly a major attraction for both occupiers and investors and we are delighted to have secured the sale to Topland Group within three weeks of agreeing terms."
Powell Lloyd acted on behalf of Topland; DTZ acted for DTZ investment management.