This study aimed to assess the cost-utility and cost-effectiveness of group-based behavioral activation with mindfulness (BAM) versus care as usual (CAU) for treating subthreshold depression in primary care. Adults aged 18 years or older with subthreshold depression were randomized into two arms and were followed up for 12 months. BAM group was provided with eight 2-h weekly treatment by trained allied healthcare workers. CAU group could access to usual medical care but did not receive extra interventions. The health service cost in the past 12 months was self-reported by the participants. Quality-adjusted Life Years (QALYs) and clinical outcome (incidence of major depressive disorder progression) were measured. Willingness-to-pay ratio for cost-utility analysis (CUA) and cost-effectiveness analysis (CEA) was US$50,000 per QALY and US$20,000 per prevented major depression case, respectively. These ratios were used in the cost-effective acceptability curve analyses to estimate the probability of cost-effectiveness of the estimated incremental cost effectiveness ratios (ICER) of BAM versus CAU. A total of 115 and 116 participants were included in the BAM group and CAU respectively. The estimated CUA ICER was US5,979 per QALY and had a probability of 0.93 that BAM was cost-effective when compared to CAU. Furthermore, when compared to CAU, BAM was cost-effective in preventing progression of major depression: the estimated CEA ICER was US$1046 per preventable case of major progression with a probability of 0.99 to be cost-effective. Group-based BAM is considered as a cost-effective alternative treatment for treating subthreshold depression by preventing major depressive disorder.