Upfield, which is owned by private equity firm KKR, produces brands including Flora, Flora Proactive, Bertolli, Story and Meadow.
Clarion’s remit covers the grocery retail and foodservice sectors, including trade media relations and the creation and implementation of PR campaigns across all of Upfield’s brands.
Clarion said it will also be "integral in communicating the business’s overall vision to create a better plant-based future".
Upfield UK and Ireland GM Steven Hermiston said the business was looking for an agency partner who "understood the demands and complexities of working across a number of brands and who would share our passion for growth".
He added: "We were very impressed with the Clarion team; they are knowledgeable, credible, enthusiastic and showed real creativity and passion for working with the Upfield business."
Clarion Communications CEO Amanda Meyrick added: "Upfield’s mission to build a better plant-based future is incredibly relevant in today’s society, and as one of the UK’s leading PR agencies with trade comms experience, we are pleased to be a part of this movement.
"The team are looking forward to working on some great campaigns and helping to establish Upfield as a leader in its category."
Last month, M&C Saatchi shop Talk.Global won a consumer PR brief to help relaunch Flora as a 100 per cent plant-based spread in the UK market.
Recently, Clarion picked up WPP-owned 'marketing implementation' agency Hogarth Worldwide as a client.