Aim The purpose of the study was to explore the test–retest reliability and minimal detectable change of selected functional fitness test items in adults with Down syndrome. Methods Forty-three adults with Down syndrome (24 men and 19 women) aged 18–50 years completed a battery of tests twice in a two-week period. The battery of tests consisted of two balance items, two flexibility items, five muscular strength and endurance items, two aerobic items, and one functional task. All items were considered valid and reliable tests in a general elderly or intellectually disabled population. The test–retest relative reliability for all repeated tests was assessed with intraclass correlation coefficient performing one-way analysis of variance. The test–retest absolute variability was measured by using the standard error of measurement (SEM) to calculate the minimal detectable change at the 90% confidence interval (MDC90). Reliability data was visualised with a Bland–Altman plot. Results All tests showed excellent intraclass correlation coefficients (ICC's>0.9). All SEM values demonstrated acceptable measurement precision (SEM<SD/2). Values for MDC90 are provided for all 12 tests. The analyses indicated that there was no major systematic bias in the plots. The scatter around the Bland–Altman was distributed randomly. Conclusion All twelve functional fitness tests demonstrated adequate feasibility and relative and absolute test–retest reliability in adults with Down syndrome in South Africa. Information of this nature will help to monitor performance alterations over time and success of training interventions.