Background: The serratus posterior inferior (SPI) muscle originates from the spinous process of T11-L2 and inserts at the lower border of the 9–12th ribs. This muscle is involved in thoracolumbar rotation and stability. Several positions can be used to improve trunk stability; the quadruped position is a good position for easily maintaining a neutral spine. In particular, during one arm lifting, various muscles act to maintain a neutral trunk position, and the SPI is one of these muscles. If trunk stability is weakened, uncontrolled trunk rotation may occur at this time. Tape can be used to increase trunk stability. There have been no studies on the effect of taping applied to the SPI muscle on thoracolumbar junction (TLJ) stability. Objects: This study compared the TLJ rotation angle between three different conditions (without taping, transverse taping, and SPI muscle direction taping). Methods: Thirty subjects were recruited to the study (18 males and 12 females). The TLJ rotation angle was measured during one arm lifting in a quadruped position (ALQP). Two taping methods (transverse and SPI muscle direction taping) were applied, and the TLJ rotation angle was measured in the same movement. Results: SPI muscle direction taping significantly reduced TLJ rotation compared to that without taping (p < 0.001) and with transverse taping (p < 0.001). There was a significant difference in the TLJ rotation angle between transverse taping and SPI muscle direction taping (p < 0.017). Conclusion: SPI muscle direction taping reduces the TLJ rotation angle during ALQP. Therefore, SPI muscle direction taping is one method to improve TLJ stability and reduce uncontrolled TLJ rotation during ALQP.