Purpose: The purpose of this study was to assess the effect of potassium iodide (KI) after applying silver diamine fluoride (SDF) on the staining of demineralized dentin covered or not by a composite resin layer. Methods: Dentin blocks from 30 bovine incisors were demineralized and randomly allocated in three groups (N equals 10): (1) control (no treatment); (2) treated with SDF; and (3) treated with SDF and KI. Half of the specimens of each group received a composite resin restoration immediately after treatment. A colorimetric evaluation, according to the CIE L*a*b* system, was performed at baseline and after seven, 14, 30, and 60 days. The ΔE data were analyzed using the generalized linear model (Δ equals 0.05). Results: The use of KI immediately after applying SDF decreased the dentin staining at all assessment times. SDF treatment only stained the dentin under composite resin after 60 days. The application of KI reduced the dentin under composite resin staining as ΔE values were similar to the control group even after 60 days. Conclusions: The use of potassium iodide minimizes the darkening of dentin and prevents the staining of the dentin under composite resin restorations in the long-term.