The agency will work on implementing social media activity for Metro.co.uk as part of the retained 12-month brief.
Diffusion has been given the responsibility of building awareness of Metro's online offering to the 3.5 million consumers who read the paper daily in 33 cities across the UK.
The campaign will look to make Metro.co.uk a destination site for 'City clickers' - 18- to 44-year-old young urban professionals looking for entertainment, showbiz and sports news.
Diffusion's task will be to increase online engagement and conversations between the paper and its readers.
The agency will target social media platforms and build relationships with key bloggers.
As part of the brief, Diffusion will promote Metro's online services, which includes the launch of its World Cup Fantasy Football game next week.
As owner Associated Newspapers looks to turn the website into a cash cow, Diffusion will also set about creating new digital marketing opportunities for advertisers.
Metro head of marketing Abi Slater said: 'Metro.co.uk has established a strong reputation for fun and irreverent content that sets it apart from traditional newspaper websites. While the newspaper clearly owns the mornings, the website has the potential to give people more Metro moments throughout the day and week.
'Sustained online consumer PR and strategic use of social media channels will be key to building awareness, traffic and consumer loyalty to Metro.co.uk.'
Separately, high street food outlet Pret A Manger has called in Fishburn Hedges to help build the brand's reputation, focusing on digital and social media. The agency will reportdirectly to Pret's CEO Clive Schlee and brand and strategy director Alex Dower.
Fishburn Hedges CEO Simon Matthews said: 'Pret has a fantastic reputation for service, its ethical approach and relationship with customers. Our primary job will be to help build that relationship online.'