Agentive steadfastness is a hitherto unarticulated and unmeasured construct, although clinicians may have drawn intuitively on it in anticipating clients' prognosis and anticipated responsiveness to adverse events. Following the conceptualisation and articulation of the agentive steadfastness construct and a measure thereof, the current study examined the validity and reliability of the agentive steadfastness index (ASI) among responding adult social media users (n = 511). Results confirmed convergent validity between agentive steadfastness and closest related psychological constructs, which were resilience (r = .715) and character strength (r = .704). Its discriminant validity was observed with other related but notably distinct psychological constructs, which were anxiety (r = −.599) and ego‐strength (r = −.244). Temporal stability was confirmed over a period of 6 months (r = .763). The ASI showed good internal (Cronbach alpha = .937) and split‐half reliability (r = .838) and a low standard error of measurement of 7.57 points within a theoretical range of 190 points. These results suggest that the ASI is a valid and a reliable measure of agentive steadfastness. Equipped with the ASI, further research is enabled on agentive steadfastness as a psychotherapeutic target and its relations with various aspects of personality, prognosis and adversity.