Background: Preoperative anxiety is very common in adults awaiting all types of surgical procedures, which can interfere with the start and completion of surgery and increase the risk of postoperative complications. In this context, many researchers and clinical practitioners have attempted to use aromatherapy to help adults reduce preoperative anxiety. Objective: To investigate systematically the efficacy of aromatherapy on preoperative anxiety in adults. Design: A systematic review and meta-analysis of randomized controlled trials was undertaken. Data sources: PubMed, Embase, Cochrane Library, Web of Science, PsycINFO and CINAHL were comprehensively searched from their inception to June 2020. Review methods: Studies on the use of aromatherapy for treating preoperative anxiety in adults were included. Two reviewers assessed the risk of bias of the included studies independently using the Cochrane Collaboration 'Risk of Bias' tool. Overall meta-analysis and five subgroup analyses regarding the type of control, the type of aroma preparations, delivery mode, session length and surgery type were performed using RevMan 5.3. The Higgins' I2 (%) statistic was used to measure heterogeneity. A funnel plot and Egger's test were used to evaluate publication bias. Results: Twenty studies comprising 1717 participants were included. The meta-analysis indicated that aromatherapy could significantly ameliorate preoperative anxiety (standard mean difference=-0.57, 95% confidence interval [-0.75, -0.39], p<0.00001) for adults awaiting surgical procedures, compared to placebo control, conventional care and no intervention. The subgroup analysis based on the surgery type showed that aromatherapy had a great improvement on preoperative anxiety for patients awaiting cardiac surgery and relatively low-risk surgery. Lavender oil, citrus species preparations and rose oil were the three most commonly used aroma preparations that could significantly improve preoperative anxiety. Most delivery modes of aromatherapy, including inhalation, massage, and oral administration, were effective. Moreover, short length (≤20 min per session) aromatherapy showed a better effect on preoperative anxiety (mean difference=-5.84, 95% confidence interval [-8.09, -3.59], p<0.00001) than aromatherapy lasting more than 20-minutes per session (mean difference=-2.15, 95% confidence interval [-3.81, -0.49], p = 0.01). Conclusion: Aromatherapy is an effective intervention for reducing preoperative anxiety in adults. Short-length aromatherapy inhalation seems to be more worthy of being recommended in clinical settings. More well-designed randomized controlled trials containing a wider range of surgical types and participants from more countries are needed to verify our findings before we can make strong recommendations.