Background: The aim of this study was to evaluate the adaptation of a resin-modified glass ionomer to the internal walls of dentin with different conditioning systems, with or without the application of silver diamine fluoride (SDF) and potassium iodide (KI). Methods: Cervical standardized cavities were prepared on the buccal and lingual sides of 15 extracted molars. Molars were then sectioned longitudinally in a buccolingual direction, obtaining two samples from each tooth with two cavities each (60 samples). Samples were randomly divided into three groups (n = 20). Each group was divided into two subgroups. One subgroup did not receive dentin conditioning, one was conditioned with 25% polyacrylic acid—10 s (PA), and one with 37% orthophosphoric acid—5 s (OPA). In the other subgroup, dentine was treated with SDF/KI and not conditioned or conditioned with PA or OPA. All cavities were filled with Riva Light Cure® (RLC). The adaptation of the RLC to the cavity walls was evaluated by SEM at 100× magnification. The area of maximum interfacial gap was magnified at 1000× to measure its size. Kruskal–Wallis and Mann–Whitney U test were used for comparison. A significance level of α = 0.05 was used. Results: No significant differences in the percentage of well-adapted samples were found in subgroups when SDF/KI was used, regardless of whether conditioner was used or not and whether PA or OPA was used (p > 0.05). Regardless of the gap size, dentin treatment with SDF/KI did not negatively influence ionomer adaptation to dentin walls significantly (p > 0.05), except for the subgroup conditioned with OPA (p < 0.05). Furthermore, it improved the adaptation in the axial wall of the subgroup without dentin conditioning (p < 0.05). Conclusion: According to the results of the present in vitro study, the use of SDF/KI did not affect RLC adaptation to the cavity walls. Subsequent use of a conditioner worsens the adaptation of the material to the cavity walls.