Understanding cub growth and developmental milestones is one crucial key in determining the success of conservation breeding of the endangered Bornean sun bear (Helarctos malayanus euryspilus). One Bornean sun bear was successfully hand-raised from 9 days old until weaning at the Bornean Sun Bear Conservation Centre, Malaysia in 2021. Maternal–cub interaction was recorded for the first 8 days before separation for hand-rearing because of a wound infection. This paper provides the details of the growth rate, milk consumption, behavior, and developmental milestones of the hand-reared cub. Health complications such as septicemia, gastrointestinal illness, and hair graying arose during the hand-rearing but were treated successfully with medicines and dietary supplements. This information can also serve as a crucial reference in the age estimation of rescued sun bear cubs from illegal pet trading and thereby facilitate cub management in captivity.