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.

Behavioral activation (BA) interventions systematically encourage positive and value-based activities. Engaging in them is an effective way to counteract negative affect, but it is unknown whether there are subtypes of activities that may have differential effects on mood. This study investigated the factorial structure of 99 potentially rewarding activities used in an online BA intervention during the COVID-19 lockdown. About 3624 German-speaking participants evaluated a list of 99 activities that were easy to apply. We analyzed the initially 99 activities by means of confirmatory factor analysis. Since activities can either be seen as reflective or formative indicators, a reflective as well as a formative model was analyzed. Although the range of chosen activities differed clearly between respondents, a one-factor model provided the best fit. It seems that a general “activity” factor is more important for explaining whether people choose a certain activity or not, than specific characteristics of the activity itself.