Gingivitis can trigger gingival diseases such as periodontitis. Since the complete removal of microbial plaques by mechanical procedures is not conceivable in some conditions and also chemical mouthwashes have a lot of side effects, finding a new treatment strategy would be useful. In the present study, for the first time, the effects of oral nano‐curcumin on gingival inflammation in patients with gingivitis and mild periodontitis were assessed. Forty eight patients with gingivitis and mild periodontitis participated in this clinical trial. In one group the patients were treated with Sina curcumin capsules 80 mg and the other group received a placebo. Clinical parameters, including modified gingival index, papillary bleeding index, and plaque index were determined on days 0, 7, 14, and 28. There were no significant differences in age, sex, papillary bleeding index (PBI), and modified gingival index (MGI) between the two groups at baseline. There was a dropout of two patients (both from the placebo group). The MGI and PBI have a significantly decreasing trend in both case and control groups and the decreases were severe in the case group. The differences between PBI and MGI in the two groups were significant at 14 and 28 days. The plaque index did not significantly change in either group over the study period. The trend of changes in plaque index was not different between the two groups of the study. In the current study, no side effect was found in the patients. Oral nano‐curcumin has positive effects on the decrease of inflammation and gingival bleeding in patients with gingivitis and mild periodontitis. Nano‐curcumin capsules have a systemic target site with more bioavailability than topical forms.