Background: Theories can provide a foundation to explain behavior, investigate relationships, and to predict the effect of interventions. The aim of the study was to clarify the use of theories in studies testing interventions to promote self-care. Method: A scoping review. PubMed, EMBASE, PsychINFO, and CINAHL were searched from January 2008 through January 2019. Nine common chronic conditions were included. We included studies testing a self-care intervention if they used a randomized controlled trial design. The study was registered in PROSPERO (#123719). Results: The search retrieved 9309 potential studies, of which 233 were included in the review. In total, 76 (33%) of the 233 studies used a theory and 24 different theories were used. Bandura’s social cognitive theory was the most frequently used (48 studies), but 22 other theories were used in a minority of studies. Most studies used theories minimally to justify or provide a rationale for the study, to develop the intervention, to select outcomes, and/or to explain the results. Only eight studies fully used a theory in the rationale, intervention development, choice of outcomes, and discussion. Conclusion: The use of theories to guide self-care research is limited, which may pose a barrier in accumulating knowledge underlying self-care interventions.