Anxiety Disorders often show a chronic course, even when treated with one of the various effective treatments available. Lack of treatment effect could be due to Treatment Resistance (TR). Consensus on a definition for TR Anxiety Disorders (TR-AD) is highly needed as currently many different operationalizations are in use. Therefore, generalizability in current TR-AD research is suboptimal, hampering improvement of clinical care. The objective of this review is to evaluate the currently used definitions of TR-AD by performing a systematic review of available literature. Out of a total of n = 13 042, 62 studies that operationalized TR-AD were included. The current review confirms a lack of consensus on TR-AD criteria. In 62.9% of the definitions, TR was deemed present after the first treatment failure. Most studies (93.0%) required pharmacological treatment failures, whereas few (29.0%) required psychological treatment failures. However, criteria for what constitutes “treatment failure” were not provided in the majority of studies (58.1%). Definitions for minimal treatment duration ranged from at least 4 weeks to at least 6 months. Almost half of the TR-AD definitions (46.8%) required elevated anxiety severity levels in TR-AD. After synthesis of the results, the consensus definition considers TR-AD present after both at least one first-line pharmacological and one psychological treatment failure, provided for an adequate duration (at least 8 weeks) with anxiety severity remaining above a specified threshold. This definition could contribute to improving course prediction and identifying more targeted treatment options for the highly burdened subgroup of TR-AD patients.