<p>Behaviour plays an important role in shaping population and community dynamics, influencing species coexistence and responses to environmental change. In particular, boldness (the tendency to take risks) and exploratory behaviours can influence resource acquisition and therefore competitive success, but these behaviours remain understudied in co-occurring marine invasive species. The Asian shore crab, <i>Hemigrapsus sanguineus</i>, and European green crab, <i>Carcinus maenas</i>, co-occur as invasive species in coastal areas of the northwestern Atlantic Ocean, including Atlantic Canada with <i>H. sanguineus</i> first observed in 2017. Both crabs use rocky intertidal habitat and consume similar prey, likely competing for resources. This study compared the boldness of co-occurring <i>H. sanguineus</i> and <i>C. maenas</i> in Atlantic Canada (44.145275, -66.193882; 2024) by investigating their tendencies to leave preferred rocky habitat with shelter to explore an open and brightly lit test arena. Their exploration was also compared by evaluating their tendency to spontaneously alternate in a plus-maze (visit the least recently explored locations in a novel environment), an indicator of exploration efficiency. While the crabs took the same amount of time to leave the sheltered habitat, <i>C. maenas</i> spent less time sheltering, suggesting greater boldness based on this task. Both crabs also tended to spontaneously alternate, but <i>H. sanguineus</i> showed a greater tendency to do so than <i>C. maenas</i>, revealing, for the first time, significant differences in spontaneous alternation behaviour between co-occurring invasive species. If <i>H. sanguineus</i>’ strong exploration ability can enhance its capacity to find resources, it may have a competitive advantage over <i>C. maenas</i> in this study region.</p>