Background: Although internet-based and mobile-based stress management interventions (iSMIs) may be a promising strategy to reach employees suffering from high chronic stress, it remains unknown whether participants with high symptom severity of depression or anxiety also benefit from iSMIs or should be excluded. Objective: This study aimed to evaluate the efficacy of iSMIs in subgroups with high symptom severity and to test whether baseline symptom severity moderates treatment outcome. Methods: Data from three randomized controlled trials (N=791) were pooled to identify effect modifiers and to evaluate efficacy in subgroups with different levels of initial symptom severity. The outcomes perceived stress (Perceived Stress Scale, PSS), depression severity (Center for Epidemiological Depression Scale, CES-D), and anxiety (Hospital Anxiety and Depression Scale, HADS) symptom severity were assessed at baseline, 7-week postassessment, and 6-month follow-up. Potential moderators were tested in predicting differences in the change of outcome in multiple moderation analyses. Simple slope analyses evaluated efficacy of the iSMI comparing the intervention group with the waitlist control group in subgroups with low, moderate, and severe initial symptomology based on means and SDs of the study population. In addition, subgroups with clinical values of depression (CES-D=16) and anxiety (HADS=8) at baseline were explored, and response rates (RRs; 50% symptom reduction) and symptom-free (SF) status (CES-D<16, HADS<8) were reported. Results: Individuals with high stress (PSS=30), depression (CES-D=33), anxiety (HADS=15), and emotional exhaustion (MBI=5.6) benefited significantly from the intervention with great reductions of stress (dpost=0.86-1.16, dFU=0.93-1.35), depression (dpost=0.69-1.08, dFU=0.91-1.19), and anxiety (dpost=0.79-1.19, dFU=1.06-1.21), and effects were sustained at 6-month follow-up. Symptom severity moderated treatment outcomes, as individuals with higher symptom severity at baseline benefited significantly more from the intervention than individuals with lower symptom severity. Furthermore, 82.9% (656/791) of individuals had clinical depression values at baseline, of which significantly more individuals in the intervention group reached at least 50% symptom reduction or fell under clinical cut-off (RR: 29.2%, 93/318; SF: 39.6%, 126/318) compared with the waitlist control group (RR: 8.0%, 27/338; SF: 18.6%, 63/338) at postassessment. Significantly more individuals with clinical anxiety values at baseline (HADS=8, 85.3%, 675/791) in the intervention group achieved at least 50% symptom reduction or fell under clinical cut-off (RR: 27.7%, 94/339; SF: 39.8%, 135/339) compared with the WLC (RR: 4.8%, 16/336; SF: 15.5%, 52/336). Conclusions: Highly burdened individuals benefit greatly from iSMIs and therefore should not be excluded from participation. Stress management may be a valid entry point to reach highly burdened individuals who otherwise may not seek treatment. Trial Registration: 1) German Clinical Trials Register DRKS00005112; https://www.drks.de/DRKS00005112 (Archived by WebCite at http://www.webcitation.org/6zmIZwvdA); 2) German Clinical Trials Register DRKS00005384; https://www.drks.de/ DRKS00005384 (Archived by WebCite at http://www.webcitation.org/6zmIerdtr); and 3) German Clinical Trials Register DRKS00004749; https://www.drks.de/DRKS00004749 (Archived by WebCite at http://www.webcitation.org/6zmIjDQPx).