Goals are often at the basis of human actions. As an essential mechanism of behavioural adaptation, individuals need to be able to flexibly implement new task goals so as to alter their actions (switch tasks) in response to contextual changes. The present study investigated the effect of acute psychosocial stress on cognitive control processes of flexible task-goal implementation with temporal focus on the occurrence interval of the hypothalamus-pituitary-adrenal (HPA) stress response. For this, forty-eight healthy volunteers were either challenged with a standardised stress-induction protocol (the Trier Social Stress Test) or underwent a standardised control situation. Subsequently, they were exposed to a task-switching procedure with two tasks alternating in random order. Participants of the stress group displayed increased salivary α-amylase activity immediately after stress exposure as well as elevations of salivary cortisol from 10 min after stress cessation, reflecting the typical stress-related activity increases in the sympathetic nervous system and the HPA axis, respectively. At the time interval of elevated cortisol levels, stressed individuals persistently showed larger performance differences between task switches and task repetitions (switch costs) than controls. This effect was reliably evident when tested 5-20 min as well as 25-40 min following treatment cessation. These results indicate that acute psychosocial stress impairs cognitive control processes of flexible task-goal implementation essential for voluntary goal-directed behaviour.