Background: Instrument-assisted soft tissue mobilization (IASTM) is thought to alter fluid dynamics in human soft tissue. The aim of this study was to investigate the influence of IASTM on the thoracolumbar fascia (TLF) on the water content of the lumbar myofascial tissue. Methods: 21 healthy volunteers were treated with IASTM. Before and after the procedure and 5 and 10 minutes later, lumbar bioimpedance was measured by bioimpedance analysis (BIA) and TLF stiffness by indentometry. Tissue temperature was recorded at the measurement time points using an infrared thermometer. Results: Bioimpedance increased significantly from 58.3 to 60.4 Ω (p < 0.001) at 10-minute follow-up after the treatment. Temperature increased significantly from 36.3 to 36.6° Celsius from 5 to 10 minutes after treatment (p = 0.029), while lumbar myofascial stiffness did not change significantly (p = 0.84). Conclusions: After the IASTM intervention, there was a significant increase in bioimpedance, likely due to a decrease in water content in myofascial lumbar tissue. Further studies in a randomized control trial design are needed to extrapolate the results in healthy subjects to a symptomatic population as well and to confirm the reliability of BIA in myofascial tissue.