'Around six' projects each year will be allocated.
Director of group media relations Paul Mylrea said agencies would support consumer initiatives similar to those encouraging use of Oyster cards or promoting station refurbishments.
The ultimate aim, he added, is 'behavioural change': more people using public transport and walking or cycling.
TfL's move comes less than three months after London
Underground set up a three-strong roster (PRWeek, 13 Jan).
TfL has used agencies including Freud Communications, Consolidated Communications and Cohn & Wolfe on projects in the past, but it now wants more formal arrangements.