Ageism might reduce the effectiveness and quality of care in older adults essential with the aging population. Although interventions to counter ageism were carried out among students, no clear integration of their results described the mechanisms underlying their impact. This study aimed to provide a comprehensive understanding of which interventions amongst undergraduate health and social services students are effective, under which circumstances, how, and with what outcomes. A realist review was carried out using 44 keywords in 6 databases. The configurations for contexts, mechanisms, and outcomes were identified for three types of interventions, considering their impact on stereotypes, prejudices, and discrimination toward older adults. In the 63 studies selected, intergenerational contact interventions exhibited the greatest proportion (14/15; 93.3%) of evidence supporting improvement in outcomes of ageism, followed by combined (67/84; 79.8%) and educational (51/67; 76.1%) interventions. Stereotypes and prejudices were challenged by the transmission of realistic and balanced information, as well as through meaningful and high-quality intergenerational contact. Meeting or hearing about a broad variety of older adults in specific conditions also helped to decrease ageism. Results highlighted the importance of seeing diversity and uniqueness in older adults, and their competencies rather than their limitations. Mechanisms were occasionally hampered by obstacles, such as induced anxiety and confirmation of negative aspects of aging. Countering ageism in health and social services students is essential to ensuring high-quality care. In knowing that educators, professionals, and institutions have a direct influence on students, their awareness of ageism should be enhanced by interventions targeting them.