In February PRWeek reported that the drinks giant was holding a pitch for its UK corporate comms and public affairs joint account, which had also been managed by Edelman since 2010.
Following a competitive pitch, Portland was appointed to the UK account in July.
This most recent pitch is on a global scale. Diageo has confirmed that Edelman is to repitch.
Dominic Redfearn, global brand and comms director at Diageo, said: "Edelman has been our corporate agency for more than six years and has done a good job for the business. With a new corporate brand strategy to deliver, the time is right to look at the support we need moving forward.
"We have contacted the specific agencies we are interested in hearing from and will be working through the process in due course."
Diageo, which owns brands including Smirnoff Vodka, Guinness and Baileys, declined to provide more details of its new corporate brand strategy for the time being.
Edelman was unable to respond at the time of publication.