Background/Objective Core muscle endurance (CME), which is the ability of sustaining the activity of trunk muscles, has been shown to be lower in patients with ankylosing spondylitis (AS). The aim was to investigate the possible relationship between CME times and balance, fatigue, physical activity (PA) level, and thoracic kyphosis angle. Methods Fifty-one patients with AS with a mean age of 41.0 years (interquartile range, 25/75 years; 29.0/51.0 years) were included in the study. Core muscle endurance times were assessed by using trunk extension, trunk flexion, and side bridge tests. Overall stability index, anteroposterior stability index, mediolateral stability index, and limits of stability were evaluated with the Biodex Balance System. Fatigue and PA levels were surveyed using Fatigue Severity Scale and International Physical Activity Questionnaire, respectively. Thoracic kyphosis angle was measured by using a digital inclinometer. Additionally, CME times were compared for "high-fatigue" versus "low-fatigue" and as "low PA" versus "moderate/high PA" groups. Spearman correlation coefficients and Mann-Whitney U test were used for statistical analysis. Results Significant correlations were detected between overall stability index, anteroposterior stability index, Fatigue Severity Scale, International Physical Activity Questionnaire, and all CME tests (p < 0.05) and between mediolateral stability index and side bridge test (p < 0.05). Limits of stability correlated only with side bridge test (p < 0.05). Core muscle endurance significantly differed between high-fatigue and low-fatigue groups (p < 0.05), except trunk flexor test (p > 0.05). No significant differences were observed between low PA and moderate/high PA groups (p > 0.05), except side bridge test (p < 0.05). Conclusions Core muscle endurance times seem to be related to PA level, fatigue, and balance but not with thoracic kyphosis angle. Assessing CME in patients with AS might help in planning individualized exercise programs.