Background: Adults with an intellectual disability (ID) have low cardiorespiratory fitness (CRF). Low CRF has been associated with a high risk of cardiovascular disease and all-cause mortality. Participation in regular exercise can help adults with ID increase their CRF. Objective: To perform a systematic review and meta-analysis of published, peer-reviewed clinical trials that evaluated the effects of aerobic exercise (AE) interventions on CRF in adults with ID, ages 18-65 years. Methods: English-language articles were searched up to June 2021 from 11 electronic databases. Data were extracted using an author-developed form. Two independent authors assessed the risk of bias using the Tool for the Assessment of Study Quality and reporting in Exercise (TESTEX). Meta-analysis was performed using the RevMan 5.3. Results: Of the 1870 article titles and abstracts screened, 16 articles were included. The average TESTEX score (out of 15) was 8.1 (SD = 3.5, range 2-14). The pooled effect was statistically significant (SMD = 0.41, 95% CI: 0.19 to 0.63, z = 3.59; p = .000) with moderate heterogeneity (I2 = 35%, p = .000). Both types of intervention produced statistically significant CRF gains, with interventions that combined AE with resistance, balance, and/or flexibility exercises being slightly more effective (SMD = 0.40, 95% CI: 0.11 to 0.70, p = .007) than non-combined interventions (SMD = 0.42, 95% CI: 0.05 to 0.79, p = .02). Heterogeneity was moderate but non-significant for both types of intervention. Conclusions: The review supports the use of AE interventions in promoting CRF in adults with ID. The interpretation is limited by the quality of evidence and by poorly described and/or executed familiarization and measurement protocols.