Despite wide recognition of workplace bullying’s harmful individual, team, and organisational effects, previous work has traditionally conceptualised it as a dyadic problem between the perpetrator and dyad. In past years, researchers have begun to examine bullying as a social phenomenon in which third parties, known as bystanders, are also involved. However, there has been to date no validated scale to measure workplace bullying bystanders despite the growing number of studies that suggest that bystanders can engage in a multitude of behaviours with different effects. To address this, we draw on existing frameworks of workplace bullying bystanders and propose four bystander responses categorised along two dimensions Constructive to Destructive and Active to Passive, resulting in four bystander ‘types’: Active Constructive, Active Destructive, Passive Constructive, and Passive Destructive, for which we develop clear definitions. Drawing from subject matter experts and past academic research, we developed and finalised the Bystander Typology Scale (BTW), consisting of 34 items that capture the four bystander response groups. Across five samples, we test the BTS’ psychometric properties and its content, discriminant, and nomological validity. Specifically, in Study 2, we test various individual differences that may predict bystander responses, and, in Study 3, we show that the BTS can be used to predict target-reported outcomes of experiencing workplace bullying. We believe that the BTS is a reliable measure that can be used by academics and practitioners to help understand the role and effect of bystanders in workplace bullying, thereby helping to reduce and prevent its occurrence.