Background: Internet-based cognitive behavioral therapy (ICBT) for tinnitus is an evidence-based intervention. The components of ICBT for tinnitus have, however, not been dismantled and thus the effectiveness of the different therapeutic components is unknown. It is, furthermore, not known if heterogeneous tinnitus subgroups respond differently to ICBT. Aims: This dismantling study aimed to explore the contribution of applied relaxation within ICBT for reducing tinnitus distress and comorbidities associated with tinnitus. A secondary aim was to assess whether outcomes varied for three tinnitus subgroups, namely those with significant tinnitus severity, those with low tinnitus severity, and those with significant depression. Methods: A parallel randomized controlled trial design (n = 126) was used to compare audiologist-guided applied relaxation with the full ICBT intervention. Recruitment was online and via the intervention platform. Assessments were completed at four-time points including a 2-month follow-up period. The primary outcome was tinnitus severity as measured by the Tinnitus Functional Index. Secondary outcomes were included for anxiety, depression, insomnia, negative tinnitus cognitions, health-related quality of life, hearing disability, and hyperacusis. Treatment engagement variables including the number of logins, number of modules opened, and the number of messages sent. Both an intention-to-treat analysis and completer's only analysis were undertaken. Results: Engagement was low which compromised results as the full intervention was undertaken by few participants. Both the ICBT and applied relaxation resulted in large reduction of tinnitus severity (within-group effect sizes d = 0.87 and 0.68, respectively for completers only analysis), which were maintained, or further improved at follow-up. These reductions in tinnitus distress were greater for the ICBT group, with a small effect size differences (between-group d = 0.15 in favor of ICBT for completers only analysis). Tinnitus distress decreased the most at post-intervention for those with significant depression at baseline. Both ICBT and applied relaxation contributed to significant reductions on most secondary outcome measures, with no group differences, except for a greater reduction of hyperacusis in the ICBT group. Conclusion: Due to poor compliance partly attributed to the COVID-19 pandemic results were compromised. Further studies employing strategies to improve compliance and engagement are required. The intervention's effectiveness increased with initial level of tinnitus distress; those with the highest scores at intake experienced the most substantial changes on the outcome measures. This may suggest tailoring of interventions according to tinnitus severity. Larger samples are needed to confirm this.