Incumbent agency Red, which chose not to re-pitch for the business, said "a change of focus [at Twitter] resulted in the relationship drawing to a close".
The Romans, which won New Consultancy of the Year at the PRWeek UK Awards 2016, began work with the social media behemoth this month.
A statement from agency co-founders Joe Sinclair and Misha Dhanak said: "Whether it's news, sport, politics, or lols, Twitter is about what's happening right now. Creative opportunities don't come more real-time than this one."
The brief includes creative campaigns, consumer media relations, trade, content, events and influencer work, as well as developing "new ways to reenergise how consumer and media audiences engage with the platform", the agency said in a statement.
The Romans has also been tasked with "creatively exploring subcultures and media outside of traditional media". Sinclair told PRWeek the agency would "PR Twitter to shine a light on certain subcultures" - although he stopped short of naming which ones.
The Romans will also work with trade and marketing media to help explain and demonstrate new promoted products when they launch, the statement added.
In December, PRWeek reported that The Romans had been appointed by Facebook to run a series of projects throughout 2017.
The first project involved teaming up with the Tate Britain gallery in London, to encourage visitors to the Turner Prize exhibition to film their reactions and opinions of the artworks, using Facebook Live.
However, Sinclair told PRWeek the agency stopped working with Facebook after this project had concluded.
Facebook's international comms lead for its Oculus VR product is Sinead Purcell. She joined as UK comms manager last year, prior to which she worked at Red Consultancy - where she ran its Twitter client account.