<p>Subjective support-focused cognitive behavioral therapy (SS-CBT) could increase subjective support among (pre)frail community-dwelling older adults, however, the underlying mechanisms remain unclear. We conducted a randomized controlled trial, in which the SS-CBT group (<i>n</i> = 50) received SS-CBT once a week for eight weeks and the wait-list control group (<i>n</i> = 50) received no intervention. The dependent variable was subjective support, and potential mediators included self-esteem, gratitude, cognitive distortions, interpersonal competence and rumination. Measurements took place before and after the intervention. We found that interpersonal competence mediated the effectiveness of SS-CBT on both perceived support availability (<i>β</i> = 2.030, 95%<i>CI</i> = 1.164, 3.081) and perceived support adequacy (<i>β</i> = 6.37, 95%<i>CI</i> = 3.52, 9.74), while self-esteem only mediated the effectiveness of SS-CBT on perceived support availability (<i>β</i> = 1.621, 95%<i>CI</i> = 0.502, 2.843). Gratitude, cognitive distortions, and rumination neither mediated the effectiveness of SS-CBT on perceived support availability nor on perceived support adequacy. These findings highlight self-esteem and interpersonal competence as the mechanisms through which SS-CBT increases subjective support, especially perceived support availability. This implicates that the SS-CBT should exclusively emphasize the apparent active ingredients (self-esteem and interpersonal competence) to promote its efficiency.</p>