The company previously handled the activity in-house before inviting agencies to pitch for the project.
Shine is charged with increasing engagement with Cos' existing consumer audience, while attracting new customers to the brand.
Director Rose Hernu will lead the account.
The agency plans to develop a robust social strategy to maximise Cos' engagement with consumers across its global Facebook presence, ensuring the right tone of voice is implemented at all times.
Hernu will report to Atul Pathak, head of comms at Cos, who said: 'Shine demonstrated a clear understanding of our objectives and the importance of making sure we define Cos' behavioural and vocal tone around social media, while fiercely maintaining the brand identity and encouraging loyalty.'