Background <p>To assess whether plaque-disclosing agents (PDA) improve self-performed mechanical plaque removal and gingival health compared with standard instruction alone in dental students.</p> Design/Setting <p>Single-center, parallel-group randomized controlled trial (1:1), Istanbul Medipol University (Oct 2023–Jan 2024).</p> Participants <p>124 fourth-year dental students (age 20–30 years) randomized to test (<i>n</i> = 62) or control (<i>n</i> = 62).</p> Interventions <p>Both groups received standardized oral-hygiene instruction (Modified Bass; interdental cleaning guidance). The test group additionally received PDA before brushing as a visual aid; all outcomes were measured after triple rinsing.</p> Primary outcome (prespecified) <p>Between-group difference in change (Δ) in Plaque Index (PI) from baseline to week 1.</p> Secondary outcomes <p>ΔPI baseline→post-brushing and baseline→month 1; ΔGingival Index (GI) baseline→week 1 and baseline→month 1.</p> Masking <p>Outcome assessor blinded at the time of measurement (after rinsing).</p> Results <p>All 124 randomized participants completed follow-up and were analyzed. Both groups showed short-term improvement (immediate reduction in PI post-brushing; GI improvement at week 1). The primary outcome showed no significant between-group difference in ΔPI from baseline to week 1. Secondary analyses similarly found no significant between-group differences in ΔPI or ΔGI at month 1.</p> Conclusions <p>In a population with high baseline oral-hygiene proficiency, adding PDA did not confer a measurable advantage over standardized instruction alone on PI or GI change. PDA may be more informative in lay populations with lower oral-health literacy.</p> Trial registration <p>ClinicalTrials.gov, NCT06080672, registered on October 12, 2023.</p>