In Japan, it is common practice to involve grandparents in the care of children with severe motor and intellectual disabilities (SMID), as it may be difficult for nuclear families to handle such children by themselves. This study aimed to explore and describe the process through which the grandparents of children with SMID may be involved in their upbringing. Data were obtained via semi-structured interviews with 13 grandparents who had a grandchild with SMID. The collected data were examined using Yasuhito Kinoshita’s modified grounded theory approach. Altogether, 11 categories were identified from 29 themes. The results revealed two broad patterns concerning the nature of the grandparents’ involvement and their definition of grandparenthood in the context of raising a grandchild with SMID. In one process, grandparents supported the child’s family. They also learned to relate to the child and their family, despite experiencing a lack of responsiveness from the child initially. This ultimately resulted in them being inspired by their grandchild. In the other process, the grandparents left the care of their grandchild to the child’s family and chose to monitor the child’s welfare indirectly. In both the processes, the level of involvement was determined by the extent to which the family chose to ask grandparents for help. The study’s findings can be used to identify the kind of support that grandparents of children with SMID require while highlighting the role of considering the needs and intentions of the children’s families.