Objective: The aim of this systematic review was to evaluate the in vitro effects of LED-based photobiomodulation on fibroblasts and osteoblasts, with particular focus on cellular processes involved in soft- and hard-tissue regeneration. Methods: A comprehensive electronic search was conducted on 3 April 2026 in PubMed, Scopus, Web of Science, Embase, and WorldCat according to PRISMA guidelines. The analyzed outcomes included cell viability, proliferation, migration, collagen synthesis, oxidative stress, mitochondrial activity, and selected regeneration-related processes. A total of 745 records were initially identified, and 32 studies met the inclusion criteria and were included in the qualitative synthesis. Results: The biological effects of LED-PBM depended strongly on irradiation parameters, including wavelength, fluence, irradiance, exposure time, treatment schedule, and the initial condition of the cells. Most included studies focused on fibroblast models. Red and near-infrared light showed the most consistent beneficial effects, particularly by supporting fibroblast viability, proliferation, migration, mitochondrial activity, ATP production, collagen-related responses, and oxidative stress modulation. In osteoblast-related models, LED irradiation showed potential to influence cell number, metabolic activity, maturation markers, and mineralization-related outcomes; however, the number of studies was limited. Blue light demonstrated dose-dependent effects, with higher fluences reducing fibroblast metabolic activity, proliferation, or viability. Green light improved fibroblast proliferation and migration in one model but was associated with increased cell death in osteoblast-like cells. Conclusion: LED-PBM may positively modulate cellular processes involved in soft- and hard-tissue regeneration in vitro. However, the observed effects are strongly parameter-dependent, and further standardized studies are required to define optimal irradiation protocols and validate their potential clinical relevance.