Relational integration refers to the process whereby several variables are integrated within a single cognitive representation. In cognitive and developmental science, it is regarded as a central function of working memory that may contribute to the development of higher order processes, such as reasoning and fluid intelligence. In the present experiment, we administered the Relational Integration Level Assessment Task (RILAT), a novel experimental task designed to assess children’s level of relational integration, to participants aged 5–12 years, along with a measure of fluid intelligence. Results yielded age norms for the development of successive levels of relational complexity and indicated a smooth progression rather than abrupt, stage-like transitions. Relational integration was found to be closely associated with fluid intelligence, as measured by Raven’s Progressive Matrices. Analyses of age-related changes in this association revealed that the relationship was strongest during the stabilization periods for each level of relational complexity.