Semiconductor devices having reinforcement structures configured to mitigate thermomechanical stresses, and associated systems and methods, are disclosed herein. In one embodiment, a semiconductor package includes a semiconductor die and a substrate coupled to the semiconductor die. The substrate can include a base structure and a reinforcement structure at least partially within a die shadow region of the substrate. The reinforcement structure can be at least partially surrounded by the base structure. The reinforcement structure has a higher stiffness than the base structure.