Aim: To compare and evaluate the marginal adaptation of the composite resin restoration-liner-dentin (C-L-D) with bioactive liners in an open sandwich technique. Materials and methods: Class II proximal boxes were prepared, extending 1 mm below the cementoenamel junction (CEJ). Groups I and II materials were placed as liners at the gingival floor, followed by composite resin compaction in an open sandwich technique. Evaluation of composite resin-liner-dentin interfaces was determined with a scanning electron microscope (SEM) at 200x & 1000x. Results: The interfacial gap values in their descending order were as follows: group II (Micron Bioactive at L-D; 18.07 µ) > group I [resin-modified glass ionomer cement (RMGIC) at L-D; 13.46 µ] > group I (RMGIC at C-L; 2.28 µ) > group II (Micron Bioactive at C-L; 1.70 µ). However, there was no statistically significant difference between the two groups tested. Conclusion: RMGIC liner in the open sandwich technique presented lower mean marginal gap values than Micron Bioactive, which can be attributable to the material's poor handling properties, causing poor adaptation. None of the tested materials created a perfect marginal seal at the L-D interface. Clinical significance: Bioactive materials are a milestone in restorative dentistry. Various bioactive incorporations have been attempted into glass ionomers to improve their physical and chemical properties. Micron Bioactive is the only available commercial product with hydroxyapatite (HA)-modified glass ionomer. Therefore, it is crucial to evaluate its properties in terms of marginal adaptation for better clinical outcomes.