The aim of this PRISMA-compliant systematic review was to analyze the evidence pertaining to disease resolution after treatment of peri-implant diseases (PROSPERO: CRD42022306999) with the following PICO question: what is the rate of disease resolution following non-surgical and surgical therapy for peri-implant diseases in adult human subjects? A literature search to identify studies that fulfilled a pre-established eligibility criteria was conducted. Data on primary therapeutic outcomes, including treatment success, rate of disease resolution and/or recurrence, as well as a variety of secondary outcomes was extracted and categorized. Fifty-five articles were included. Few studies investigated the efficacy of different non-surgical and surgical therapies to treat peri-implant diseases using a set of pre-defined criteria and with follow-up periods of at least one year. The definition of treatment success and outcomes of disease resolution differed considerably among the included studies. Treatment of peri-implant mucositis was most commonly reported to be successful in arresting disease progression for ≤60% of the cases, whereas most studies on peri-implantitis treatment reported disease resolution occurring in <50% of the fixtures. In conclusion, disease resolution is generally unpredictable and infrequently achieved after the treatment of peri-implant diseases. A great variety of definitions have been used to define treatment success. Notably, percentages of treatment success and disease resolution were generally underreported. The use of standardized parameters to evaluate disease resolution should be considered an integral component in future clinical studies.