The agency, which has Edinburgh and Manchester offices, said it wanted to quash confusion among clients and journalists and reflect its increasingly national workload.
Biss Lancaster chairman Graham Lancaster said: 'We wanted a uniform brand.
It had become confusing for clients and journalists because Biss Lancaster and Leedex share clients.'
In addition to shared clients such as BAE Systems, the Electoral Commission and the Central Office of Information, Leedex Manchester handles national briefs for The Bank of Scotland and Whitbread, while Leedex Edinburgh works on whisky brand Bell's and charity Breast Cancer Care.
The Manchester and Edinburgh offices remain under the direction of Brian Beech, who has taken the new title of MD for Edinburgh and Manchester.
'I will not be sad to see the Leedex name go,' he said. 'Biss Lancaster is one of the best names in the industry.'
Biss Lancaster bought Leedex in 1999 from entrepreneur Kevin Morley when the agency was headed by Sarah Muirhead.
Beech continues to sit on the Biss Lancaster board with the chairman and other directors Jonathan Sanchez, Belinda Taylor and Nick Parkinson.
Euro RSCG Biss Lancaster, headquartered in London, is part of the global Euro RSCG Worldwide group.