Aim: The purpose of this study was to assess the impact of different flowable liners on cuspal deflection (CD) in extracted maxillary premolars that have been restored using bulk-fill composite. Materials and methods: A total of 48 mesio-occlusal-distal (MOD) cavities were prepared in extracted human maxillary premolar teeth. The preparations (n = 12) were randomly divided into four groups. Groups I, II, and III were lined with Neo Spectra ST, Tetric N-Ceram, and Filtek 3M liners, respectively, and all were further restored using Tetric N-Ceram bulk-fill composite. The control group, group IV, received Tetric N-Ceram bulk-fill packable composite. In this study, CD was measured using a digital micrometer, and measurements were recorded in millimeters. Results: Cuspal deflection mean values were statistically significant immediately after restoration (CD2) and post 24 hours of restoration (CD3) in all groups, with group IV, in which no liner was used, showing maximum deflection and group II showing minimal deflection compared to all other groups. The intergroup comparison showed no significant difference between groups I, II, and III, but all three groups were statistically significant when compared to group IV. Hence, restorations in which liner was used recorded less CD compared to those in which no liner was used, indicating higher polymerization stresses in that group. Conclusion: The application of a liner has a positive impact on CD, reducing it by decreasing polymerization shrinkage, distributing stress, and improving marginal adaptation. Clinical significance: Minimizing CD in dentistry prevents cohesive and adhesive restorative failures and prolongs the lifespan of dental restorations by reducing stress on them. In this study, it was proven that restorations lined with a liner recorded less deflection than those in which no liner was used.