Background: The relationship between splenic size and age has been well studied in children but reports on adults have been inconsistent. In Nigeria, the relationship between splenic size and age as well as the inter-decade changes in splenic length in adults has not been adequately studied. Objectives: This study aimed at determining the relationship between splenic length and age in adult Nigerians. Methods: This is a retrospective study in which sonographic splenic lengths of adult Nigerians aged 30 years and above were correlated with age. The relevant data were obtained from the medical records of those who had ultrasound splenic examination for the purposes of medical check-up between December 2016 and December 2017. Results: There were 50 cases aged between 34 and 57 years, with a mean age of 42.76 ± 7.73 years. Their spleens had normal shape and echotexture. The mean splenic length was 9.56 ± 1.37 cm. Splenic length correlated negatively with age and decreased from 9.79 ± 1.00 cm in the fourth decade to 9.49 ± 1.67 cm in the fifth decade and to 9.24 ± 1.63 cm in the sixth decade. The mean splenic length of 9.56 recorded in these cases was relatively small when compared to the results by other workers who studied subjects of much younger age groups. Conclusion: The studied cases which were in the fourth, fifth and sixth decades of life had a relatively small mean splenic length. There was no significant change in splenic length with increasing age.