Background Compassion-based interventions delivered over the internet are showing promising results for the promotion of psychological health and well-being. Several studies have highlighted their feasibility, acceptance, and preliminary efficacy. However, this is an incipient field of research, and to the best of our knowledge, there are no data available from Spanish-speaking countries. Objective The aim of this study is to investigate the feasibility, acceptance, and preliminary efficacy of the Internet Attachment–Based Compassion Therapy (iABCT), a web-based version of the Attachment-Based Compassion Therapy, in Spanish speakers from the general population. Methods This feasibility study features a single-arm, uncontrolled, within-group design with an embedded qualitative and quantitative process evaluation at baseline, immediately after the intervention and at the 3-month follow-up. A minimum of 35 participants from the general population will be allocated to iABCT. Feasibility measures will include attrition rate, patterns of use of the web-based system, and participants’ acceptability, usability, and opinion. The primary outcome was measured using the Pemberton Happiness Index. Secondary outcomes were measured using the Compassion Scale, Self-Compassion Scale, Forms of Self-Criticizing/Attacking and Self-Reassuring Scale-Short form, Five Facets of Mindfulness Questionnaire, Relationships Questionnaire, General Health Questionnaire, Non-Attachment Scale, International Positive and Negative Affect Schedule Short Form, Purpose-In-Life Test, and difficulties regarding the practice of compassion (Compassion Practice Quality Questionnaire). Mixed models will be used to evaluate primary and secondary outcome measures. A qualitative content analysis of the participants’ qualitative responses will also be performed. Results Enrollment started in February 2020 and will be finished in April 2020. Data analysis will start in October 2020. Conclusions To our knowledge, this study will, for the first time, show data on the feasibility, acceptability, and preliminary efficacy of web-based compassion (and self-compassion) training—that is, the adapted iABCT—in Spanish speakers from the general population. Further aspects of their implementation (ie, facilitators, barriers, and unwanted effects) and mechanisms of change will be investigated. This study will allow the revision and fine-tuning of the developed intervention, study design, and planning procedures, as well as the initiation of a future randomized controlled trial. Trial Registration Clinicaltrials.gov: NCT03918746. Registered on April 17, 2019. Protocol version 1, 6 March 2019. International Registered Report Identifier (IRRID) PRR1-10.2196/16717