Modular building structures and systems for construction of modular buildings are provided. A modular building structure according to one embodiment includes at least two panels having two outer skins and an inner core; at least two panel spacers between the outer skins of the panels; at least two panel connectors partially receiving the panels; at least one coupling bar; and at least one corner coupler configured to couple two of the panels substantially perpendicular to each other. A modular building structure may further include at least one straight coupler configured to couple two of the panels substantially parallel to each other.