In this paper, an elastic poly­(vinylidenefluoride-co-trifluoroethylene) piezoelectric yarn for the application of a muscle patch sensor is presented. The electrospinning method is used to fabricate the piezoelectric yarn, and different parameters were used to control the orientation and structure of piezoelectric fibers. We further develop a post-alignment process to reorganize the orientation of fibers and to reshape fiber microstructures. Two unique microstructures of piezoelectric fibers that have an excellent elastic performance were identified. This piezoelectric yarn is composed of skewed and crimped fibers that align along the elongation direction, and it can be cyclically stretched up to 65% strain with good linearity, durability, and repeatability. Its mechanical behavior is superior to randomly distributed and fully straightened piezoelectric fibers, and it is suitable for long-term use of larger strain sensing. Our study demonstrated that this piezoelectric yarn can be stretched for more than 12 h under a repeated 1 Hz cyclic deformation. Using this elastic piezoelectric yarn, a muscle patch sensor that can be attached to the skin over human muscles for real-time monitoring is developed. The concentric, eccentric, and isometric contractions of biceps and triceps can be measured simultaneously to study their contraction behaviors. To further verify whether this patch sensor can be used under intense exercise conditions, the contraction behavior of a soleus muscle during stationary jumping and running is monitored to demonstrate sensor performance. Finally, this patch sensor is sewed onto a chest band, and it is verified that both breathing movement and heartbeat can be monitored.