Ambient temperature affects behavioural traits, including activity and exploration levels, that partly determine feeding rate and predator avoidance, and in turn, growth and survival, especially in ectotherms. The most studied effects of temperature on behavioural traits concern how ambient temperature simultaneously affects behavioural performance. Much less is known about how temperature during early development may affect behavioural traits exhibited later in life. Here, we investigate how temperature variation from fertilization to hatching in brown trout affects behavioural traits exhibited at the time of yolk absorption and the start of exogenous feeding by conducting a laboratory experiment. We found that a just 1.5°C warmer temperature from fertilization to hatching (i.e., comparison between 3.5°C and 5.0°C treatments) made juveniles markedly less active and explorative. For salmonids, keystone species in alpine streams, embryonic development occurs during winter, when temperatures have risen in recent years, partly due to storage hydropower. Our study suggests that winter warming can reduce the activity and exploratory behaviour of trout juveniles, possibly affecting their growth and survival.