The COVID-19 outbreak strongly restricted daily activities, creating a risk factor for negative affect and depression. This study assessed the immediate effects of a behavioural activation (BA) intervention on positive (PA) and negative (NA) state affect. We expected depression and anxiety to function as moderators reducing the intervention effects. In a quasi-experimental online study, 3624 German-speaking participants evaluated a list of rewarding activities between 9 April and 26 April 2020. A subsample of 2561 (71%) additionally engaged in an imagination task. Depression, anxiety, socioeconomic variables and COVID-19 related burdens were assessed as moderators. There was an increase in PA (total sample d =.13; subsample: d =.27) and a decrease in NA (total sample d = −0.68; subsample: d = −0.71; all p <.001). The effects rose with higher levels of depression and anxiety (all p <.001). Furthermore, living with family enhanced the effects on NA, while additionally having to take care of children reduced them. An easy-to-use intervention prompting BA could improve state mood during lockdown. Participants with higher depression and anxiety benefit more. Implications for the prevention of mental health problems during a pandemic are discussed.