Abstract Background Hypochlorous acid ( HOC l) is a non‐antibiotic antimicrobial substance with significant effects on pathogenic oral micro‐organisms. The effects of HOC l as an antiplaque agent have not been studied. Objective The aim of this study was to evaluate the substantivity of HOC l mouthwashes compared with chlorhexidine ( CHX ) rinses and a placebo. Materials and Methods A double‐blind randomized controlled trial with 75 participants was conducted. Participants were divided into five groups using block randomization: 0.025% HOC l, 0.05% HOC l, 0.12% CHX , 0.2% CHX , and sterile water as a placebo. Participants were instructed to use each rinse solution for 30 seconds after dental prophylaxis. Samples of saliva were taken at baseline and after 30 seconds, 1, 3, 5 and 7 hours to assess substantivity, and bacterial viability was established by the fluorescence method. Visible plaque in all participants was assessed with the Turesky index at baseline and at 7 hours, and adverse events were also assessed. Results HOC l led to a 33% reduction in bacterial counts in the saliva after 30 seconds compared with a 58% reduction by CHX . HOC l has no substantivity, and bacterial counts returned to baseline after 1 hour. Placebo treatment led to the highest plaque count after 7 hours compared with the CHX and HOC l groups, although the differences were not significant. HOC l rinsing induced the highest percentages of unpleasant taste and dryness sensations. Conclusions HOCl rinses have an initial effect on bacterial viability in saliva but have no substantivity. Other mechanisms may explain its antiplaque effect.