The aims of this study were to investigate, firstly, the relationship between diet and disease activity and, secondly, the presence of gastrointestinal symptoms and their relationship to diet among patients with ankylosing spondylitis (AS) using a cross-sectional design. One hundred sixty-five individuals diagnosed with AS were invited to complete a self-administered postal questionnaire regarding demographic data, diet, medication, and gastrointestinal symptoms in addition to two established disease assessment questionnaires, i.e., the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and Bath Ankylosing Spondylitis Functional Index (BASFI). No significant correlation between diet and disease activity was found. Overall, 27% of the patients reported aggravating gastrointestinal problems when consuming certain foodstuff(s). The 30% of patients who reported suffering from gastrointestinal pain had significantly greater disease activity and poorer functional status according to their BASDAI and BASFI scores (p < 0.01 and p = 0.01, respectively). Patients who reported gastrointestinal pain had a significantly higher consumption of vegetables (p < 0.01) and lower consumption of milk and soured milk (p = 0.04). No significant correlation was found between the use of non-steroidal anti-inflammatory drugs (NSAID) and gastrointestinal symptoms. In multiple regression models, BASDAI and the consumption of vegetables were independent and statistically significant predictors of gastrointestinal pain. To conclude, in a group of Swedish AS patients, no correlation between diet and disease activity could be detected. There were, however, correlations between diet and gastrointestinal pain. Gastrointestinal problems were also found to be prevalent in AS, independent of NSAID usage.