Suckling behaviors are useful to better understand mother-offspring relationships. However, in many species, knowledge about nighttime suckling behavior is sparse. In the present study, we investigated suckling behavior in four calves of the reticulated giraffe (Giraffa camelopardalis reticulata) and their mother in the Kyoto City Zoo, Japan, by analyzing video clips of 9614 h. The relation between a calf's age and the mean duration and frequency of suckling were consistent with the results of previous daytime studies: the longest duration (80 ± 4 s) and highest frequency (0.41/h) of suckling occurred when the calves were young, and decreased thereafter. Suckling frequency was lower at night than during the day, probably because the mother spent much of the time resting at night. The mother initiated suckling behavior in offspring until six months of age. Most often, the mother terminated suckling except during the first few weeks after birth. Additionally, we reported that the average weaning period was 448 days (n = 5) and that the last born (sixth) continued to suckle for more than 750 days, which is a rare phenomenon. We believe the present report helps to fill the lack of knowledge about nighttime suckling behavior in giraffe.