This review assesses the extent of reported theory use in electronic health weight management interventions targeting young adults aged 18–35 years. Twenty-four eligible studies were identified. Two independent reviewers extracted data and coded for theory use using the Theory Coding Scheme. Overall, the mean total use of theory score was 6/23 (SD = 5; Min. = 0, Max. = 17); 17 studies were classified as having weak application of theory, five as moderate, and two as strong. The majority (N = 18) of studies mentioned theory, however, most (N = 14) did not report how intervention techniques related to theoretical base. No study used theory to select intervention recipients and only four used theory to tailor intervention techniques to recipients. Limited studies reported theory testing (N = 6) and no study used intervention results to build and/or refine theory. Results indicate that weight-related outcomes may be enhanced when at least one or more theoretical constructs are explicitly linked to an intervention technique and when theoretical constructs are included in evaluations. Increases in theory application and reporting are needed to assist the scientific research community in systematically identifying which theories work, for whom, how, why, and when; thereby delivering an advanced understanding of how best to apply theory to enhance intervention outcomes.