Objectives This article presents the validation of a Generic Multifaceted Automaticity Scale (GMAS) assessing the automatic properties of habitual behaviours. Design A quantitative approach was adopted within various adult samples in order to confirm the internal and external validity of the GMAS pertaining to Physical Activity (PA). Method Study 1 investigated the content validity of a series of items among experts (N~=~13) and respondents (N~=~26). Study 2 examined the scale's construct validity for PA (N~=~293). Study 3 tested the GMAS external validity (N~=~161). Study 4 evaluated the internal consistency and predictive validity for the GMAS applied to transportation modes (N~=~167). Study 5 used a 1-week prospective design and included Theory of Planned Behaviour (TPB; Ajzen, 2012) variables in order to test the predictive validity of the GMAS for active transportation (N~=~125). Results Study 1 supported the fact that the 9 items primarily reflected one facet of automaticity. Study 2 validated the hierarchical structure of the scale. In Study 3, convergent validity was confirmed regarding PA through significant correlations with the Self-Report Behavioural Automaticity Index (SRBAI; Gardner et~al., 2012) and predictive validity was supported by significant correlations with self-report PA behaviour. Study 4 confirmed its internal consistency and predictive validity. Study 5 demonstrated that the scores derived from the GMAS significantly predicted active transportation behaviour, above and beyond TPB constructs. Discussion The GMAS appears as a valid instrument for research on everyday PA behaviours.