Purpose: The purpose of this study was to characterize studies on aromatherapy and sleep quality and to identify the measurable effects of aromatherapy on the sleep quality of adults and elderly people. Methods: We searched eight electronic databases for relevant studies published between January 2011 and December 2019 using the medical subheadings, "adult or elderly," "aromatherapy or aroma intervention," and "sleep quality or sleep satisfaction." Statistical analyses of the combined effect sizes, homogeneity, heterogeneity, and trim-and-fill method were performed using MIX 2.0 Pro. Results: Based on the combined results of the 30 included studies, the overall effect size of sleep quality was 0.74, which was statistically significant. A subgroup analysis was performed to assess the overall improvement in sleep quality based on country, study design, and type of intervention: the effect size of aromatherapy in East Asia, quasi-experimental studies, and aroma massage therapy studies was 1.02, 1.24, and 1.30, respectively, with higher sleep quality in each subgroup. The effect size was 0.86 and 0.80 for ≤12 interventions and for an intervention period of ≤4 weeks, respectively. Aromatherapy interventions lasting >20 min per session had an effect size of 1.28, with no significant difference in sleep quality. Quality evaluation revealed that the sleep quality after aromatherapy was significantly high in studies with methodological quality scores >8, with an effect size of 0.93. Moreover, significant combined effects of aromatherapy were observed with other variables such as stress, pain, anxiety, depression, and fatigue. Conclusion: Aromatherapy improves sleep quality and reduces stress, pain, anxiety, depression, and fatigue in adults and elderly people.