Hexagon Care Services has taken 8,500 sq ft within Unit 1 Tustin Court in Preston Docklands basin, on 10-year lease from landlord Tustin Developments.
The office is spread over three floors of the 40,000 sq ft block.
Robert Pinkus & Co acted on behalf of the landlord.
Hexagon Care will join other tenants in the building including the Yorkshire Bank and the Shaw Trust.
Hexagon Care looks after children and young people aged 11-18 years old upon admission into the care of the local authorities and runs more than 35 core services across the country.
Richard Hargreaves, commercial property manager at Tustin Developments, said: "Tustin Court is now fully let which is great news for the existing tenants in the development and will provide an additional boost to the surrounding local economy. The offices are in a premier location overlooking the marina, surrounded by a variety of local amenities including a supermarket, 10-screen cinema and several restaurants and we wish the new tenants all the best in their new location."