Shell, which has reportedly appointed R3 to handle the process, spends an estimated $200m (£142m) on marketing annually.
The Anglo-Dutch energy company has worked with JWT since 1997, following a pitch against Shell’s other roster agencies at the time, Publicis and Ogilvy & Mather.
It first appointed MediaCom in 2001, ending a five-year relationship with CIA, shortly before CIA was merged to create MEC (now Wavemaker).
In a statement, the incumbent creative agency said: "J Walter Thompson/WPP is proud of the innovative and award-winning work we have created with Shell over the past 20 years. We see this review as an opportunity to enhance our relationship and look forward to participating."
In recent years, Shell has shifted its marketing to proving its renewable energy credentials, and focusing on growing brand awareness among younger consumers.
Last month, the brand launched the latest phase of its "#makethefuture" campaign in the form of a music video featuring five pop stars to represent clean energy initiatives.
Neither Shell nor MediaCom were willing to provide a comment.