The European mudminnow ( Umbra krameri ), a globally threatened fish species, is increasingly forced to coexist with the invasive Amur sleeper ( Perccottus glenii ), its competitor and potential predator. Invasive species often disrupt native ecosystems by altering habitat use, competition dynamics, and predation pressures. To better understand the behavioural implications of this invasion, we conducted a series of aquarium experiments comparing both species. Behavioural differences along the boldness–shyness continuum between individuals can influence competitive outcomes and survival strategies. Our first objective was to assess whether European mudminnows and Amur sleepers differ in boldness and whether prior experience with the invasive species affects the behaviour of the mudminnow. We tested individuals from populations with and without previous exposure to Amur sleepers, measuring boldness through standardised behavioural assays. These revealed no significant difference in boldness between naïve and non-naïve mudminnow populations, suggesting insufficient time for behavioural adaptation or context-dependent trait plasticity. Conversely, Amur sleepers exhibited lower boldness, possibly indicating stronger predation pressure on mudminnows. These findings highlight the complex behavioural dynamics in invaded freshwater ecosystems. Beyond individual traits, direct interactions between the species may shape the invasion outcome. Although the Amur sleeper is considered a facultative piscivore, the literature describes cases where it has preyed on the European mudminnow individuals. Therefore, our second set of laboratory experiments investigated whether the mudminnow recognises the invasive species as a potential predator. We focused on the anti-predator behaviours without physical contact between predator and prey. To ensure that observed prey behaviours were predator-avoidance responses, we compared their behaviours in the presence of an obligate predator, the northern pike ( Esox lucius ). By arranging prey in homo- and heterospecific pairs, we tested whether the presence of an Amur sleeper competitor influences the mudminnow’s anti-predator behaviour. Furthermore, using naïve and experienced mudminnow individuals, we examined whether prior exposure to the invader leads to behavioural adaptations that improve survival in invaded habitats. Our results provide insight into the consequences of Amur sleeper invasion for endangered native species, offering valuable information on resource utilization and competition outcomes, especially for limited resources. The results from our predation-related experiments shed light on how the invader’s presence influences predation pressure on native fish. Understanding these behavioural interactions is essential for assessing the long-term impact of Amur sleeper invasions on European mudminnow populations. Our study has potential conservation implications, emphasising the need for targeted management strategies to protect the European mudminnow in the face of biological invasions.