Lockdowns have been imposed around the world to contain the spread of the coronavirus pandemic. However, excessively stringent measures might be a threat to people’s mental health. This study examines the impact of the COVID-19 lockdown on psychological well-being and its relationship to habit and routine modifications in a sample of 584 participants on lockdown in Spain. Habits and routines were explored in relation to media and social media use, household chores, eating, drinking and sleeping habits, working and studying, exercise and leisure, and personal care. Participants reported an important increase in negative affect as well as an important decrease in positive affect during the lockdown period, compared to before the lockdown. The decline in psychological well-being was more pronounced in younger participants. There was also a notable increase in media and social media consumption, home cleaning and tidying up, eating and sleeping, cooking and baking, reading for leisure, talking or doing activities with other people in the home, and handwashing. Increases in media and social media use, eating, and doing nothing, were significantly associated with an increase in negative affect and a decrease in positive affect. The results contribute to understanding the impact of the lockdown on psychological well-being and its relationship to habit and routine modifications during this period.