Introduction Internet‐based cognitive behavioral therapy for depression (iCBT) has been advanced as a valuable alternative treatment option, generating promising results. However, little is known about its underlying mechanisms of change. Objective We aimed to provide an overview of the state of the art regarding the mechanisms of iCBT for adult depression, in the context of iCBT efficacy. Method We conducted a systematic qualitative review of 37 randomized clinical trials, assessed the risk of bias in the included studies, and used a systematic evaluative framework to establish the scientific status of iCBT, based on evidence regarding clinical efficacy and mechanisms of change. Results Findings indicated that iCBT mechanisms of change are clearly underinvestigated, although iCBT is relatively efficacious, at least in the short term. The quality of iCBT randomized clinical trials proved to be suboptimal. Conclusions The iCBT theory should be clearly specified and adequately investigated to design and implement highly efficacious therapeutic packages. Without considering the iCBT mechanisms of change along with iCBT efficacy, the extent to which iCBT is an empirically validated treatment remains questionable.