Core-shell fibers utilizing carbon fiber as the load-bearing element and a functional shell are in the early stages of research and development, but are already showing promising results in multifunctional composite fabrication. This study builds on prior multifunctional fiber research that applied hydrothermal synthesis to grow BaTiO3 on carbon fibers. In this work, the hydrothermal reaction conditions are thoroughly explored and illustrate the typical trade-off between the tensile strength of the core fiber and the d (33) piezoelectric strain coefficient. This trade-off exemplifies the difficulty in synthesizing multifunctional fibers that maintain mechanical integrity while offering enhanced electromechanical functionality. By studying the hydrothermal synthesis conditions, parameters are established that result in the first demonstration of tetragonal phase BaTiO3 on carbon fiber. Synthesis parameters are developed that maintain the tensile strength of the core carbon fiber while increasing the d (33) piezoelectric strain coefficient of the BaTiO3 film. The optimal fiber has a tensile strength of 4.96 GPa and a d (33) of 39.2 pm/V, which equate to 16.2 and 49.6% increases over the prior synthesis of BaTiO3 coated carbon fibers, respectively. Therefore, this work establishes hydrothermal reaction conditions that create higher-performance multifunctional fibers through the development of tetragonal phase BaTiO3 on carbon fiber.