Aim: This study evaluated the microleakage of pre-heated versus high viscosity bulk fill and injectable resin composites. Materials and Methods: Forty-eight freshly extracted premolar teeth were collected; Standardized class V cavity preparations were prepared and then divided randomly into 3 equal groups: a thermoviscous one (VisCalor Bulk), and a high viscosity resin-based composite (X-tra fil), G-ænial universal injectable composite. Different insertion techniques were applied for the three composite resins. VisCalor Dispenser was used to pre-heat the VisCalor resin composite specimens at 65⁰C, and Xtra-fil composite was applied in a bulk of 4mm. In contrast, Gaenial composite was injected directly inside the cavity. Each specimen was then submerged for a full day in a 2% methylene blue dye solution. A stereomicroscope was utilized to examine the penetration of methylene blue to evaluate the microleakage of the restorations through the use of image analysis software. The results were investigated by SPSS with significance level set at p≤0.05. Results: The mean microleakage was significantly higher for VisCalor bulk fill at 694.3 than X-tra fil at 152.7 (P=0.0145) and significantly higher than G-ænial at 219.5 (P=0.0305). The difference in mean microleakage between X-tra fil and G-ænial was not statistically significant (P=0.9103). Conclusion: Microleakage was present in all three of the composite materials that were evaluated. Pre-heating VisCalor bulk fill resin composite to 65⁰C increases marginal gap formation compared to other unheated resin composite materials. Unheated composite materials represented by X-tra fil and Gaenial had superior sealing ability and lower microleakage than VisCalor bulk fill.