Mersey Care NHS Trust has leased the V7 building located within Kings Business Park, Knowsley, from landlord Maghull Group.
The trust has taken 34,055 sq ft over two floors at the Grade A office building, relocating from its site in Maghull.
Sean McGurren, managing director of Maghull Group, said: "We already have excellent landlord & tenant relationships with NHS organisations who are tenants in several of our other office buildings in Merseyside and Manchester and we are delighted to continue this."
The building was bought by the local property company in 2007 and has recently been marketed by commercial property consultants Mason Owen.
Andrew Owen, director of Mason Owen, said: "The occupation of V7 by Mersey Care NHS Trust was one of the largest out of town office lettings of 2012 and illustrates that even in a tough market, there are still deals to be done for well located, quality buildings such as V7.
"We are delighted and extremely proud to have completed such a large and high profile letting in a time when such deals are few and far between."
Neil Smith, director of finance and performance at Mersey Care, said: "We have agreed terms that not only represent good value for money for the Trust, but also support our Estates Strategy and Carbon Reduction Programme."
Mersey Care NHS Trust was represented in the agreement of terms by CBRE.