Asacol is the market-leading treatment for inflammatory bowel disease, including ulcerative colitis and Crohn's disease.
Resolute beat incumbent Healthcare Solutions, plus Manning Selvage & Lee, in a three-way contest for the contract.
The agency switch comes as P&G plans to launch a higher-dose version of the product that could reduce the need for steroid treatment and offer patients a more convenient option.
Resolute joint managing director Paul Blackburn said his agency's hire came at a 'critical time'. He added that P&G aimed to 'positively impact outcomes for inflammatory bowel disease patients by delaying the use of steroids, thus helping to avoid side effects'.
An immediate priority for Resolute is promotion of Asacol at the World Congress of Gastroenterology in Montreal, Canada, from 10 to 14 September.
Resolute's six-strong account team is being jointly led by associate directors Dr Diane Ross and Sarah Swift.
Although Resolute has never formerly been retained by P&G, the agency previously worked on a project for Asacol, promoting it at the British Society of Gastroenterology AGM in Birmingham in March.
P&G will continue to work with Healthcare Solutions, which had held the Asacol account for almost three years, on a project basis. Healthcare Solutions said planning for one such campaign was currently in its final stages.
Resolute reports to P&G brand manager Nicolas Rouzioux.
Rival products to Asacol include Shire Pharmaceuticals' treatment Colazide, which is promoted by HSD Communications.