The pitching process is understood to be at an early stage, with a request for proposals having been sent out only recently.
Sources said details about the brief and scope of work are scant as yet, but the business has indicated it is looking for a consumer agency patner.
National Express declined to comment about the review.
The company, which has worked with creative agency One Black Bear since 2016, released the ‘Who knows what you’re going home to’ campaign late last year.
National Express is the biggest coach operator in the UK and Spain, and the second-largest school bus operator in the US.
In the UK, the company's coach network serves 650 destinations.