Undisclosed agency is considered anomalous by many and often explained on the basis of commercial convenience. Although some aspects are similar to disclosed agency, the significant differences between them have given rise to rules peculiar to undisclosed agency. The content and basis of some of these rules are uncertain even if they appear reasonable. This paper suggests that the rules relating to the principal‐third party relationship that are peculiar to undisclosed agency can be placed on a stronger doctrinal basis if such rules are understood as contractual terms that are implied in law (mainly). Such analysis is consistent with and supports the broadly accepted rules that govern the relationship between the undisclosed principal and third party, and can provide a useful framework within which to resolve those aspects of the relationship that are uncertain or controversial.