IM Properties, the owner of the historic building in Birmingham city centre, is currently carrying a major renovation project following the departure of previous tenant Wragge & Co in 2014.
Pinsent Masons will move from its base in the Wesleyan building in Colmore Circus in 2017 to coincide with its lease expiring to take more than 40,000 sq ft over two floors.
Greg Lowson, head of Pinsent Masons in Birmingham, said: "The West Midlands remains a crucial market for us and this is a significant moment given our roots in Birmingham dating back to 1836.
"The opening of Grand Central and the impending construction of the rail college and the HS2 station are just a few examples of the significant infrastructure spend that the city is to benefit from over the next decade.
"Birmingham is going to be among the most vibrant legal and commercial centres outside of London....Given that context we are proud to be able to offer access to an internationally recognised and innovative legal service in the heart of the Midlands."
John Hammond, UK investment director at IM Properties, added: "They are exactly the calibre of tenant we had set our sights on when we pressed the button on the £30 million redevelopment of the building last September.
"Birmingham is an exciting place to be right now and we are pleased that our show of commitment to the region is proving justified."
Pinsent Masons said it planned to open a second base in the city in due course to house existing personnel who provide operational services to the firm across the UK and internationally.