Background: One of the significant changes in childrearing over the past 20 years was the increasing number of grandparents raising their grandchildren. Being a primary caregiver to help nurture grandchildren is considered a principal occupation for many older adults. Grandparents frequently play a significant role in helping their families care for grandchildren diagnosed with autism spectrum disorder. However, little was known about the grandparents of these children in Thailand. Objectives: This phenomenological qualitative research aimed to examine the forms and meanings of grandparenting for grandchildren with autism spectrum disorder aged 2-6 years. Materials and methods: Semi-structured interviews were conducted. Seven participants who were grandmothers of children with autism spectrum disorder participated in this study. Data were transcribed verbatim, and the transcripts were coded and analyzed using a content analysis method. Data were interpreted using occupational therapy and occupational science notions to address the research questions and discussion. Results: Three main themes were developed. The first theme was reasons for deciding to become a primary caregiver. Because of the participants’ involvement in caring for grandchildren, the families could face this difficult task together, and grandchildren would be effectively raised by grandmothers, receiving the appropriate care. The second theme revealed three forms of grandparenting occupations, which involved time allocation to fit daily demands, adjusting childrearing methods to promote the development of a grandchild, and dealing with grandchildren’s behaviors caused by autism spectrum disorder symptoms. The last theme was the meanings of grandparenthood. Participants addressed what it meant to be grandparents raising grandchildren diagnosed with ASD through their duties and commitments, and love bonds. Conclusion: With a better understanding of how grandparenting occupations were managed and orchestrated to meet the needs of families and grandchildren with ASD, we could see the adaptation process in childrearing methods. Moreover, the findings suggested that engagement in grandparenting occupations was meaningful for the participants in this study.