There is a need for instruments designed for patients with asthma to self-report their performance of inhaling steps. We aimed to develop an accessible and easy-to-use patient-reported tool for inhaler technique assessment, which could also serve as a training and monitoring resource for any type of inhaler device, and to evaluate its feasibility, validity, and reliability in adults with asthma. The development was based on literature review and pilot testing with clinicians and patients. The Inhaler Technique Questionnaire (InTeQ) asks about the frequency of performing five steps when using inhalers (on a five-point Likert scale). We analyzed data from adults with persistent asthma (n = 361). We examined the measurement model using Mokken scaling analysis, construct validity by assessing hypotheses on expected discrimination among known groups, and reliability based on internal consistency and reproducibility. Means of the InTeQ items were in the range of 0.23-1.61, and coefficients of homogeneity were above the cutoff point, demonstrating the unidimensionality of the scale. Known groups' global score differences were statistically significant between patients reporting having "Discussed in detail" or having "Not discussed/Only in general" the inhaler technique with their healthcare providers (p = 0.023). The Cronbach's alpha coefficient was 0.716, and the intraclass correlation coefficient was 0.775. The InTeQ is a feasible, valid, and reliable instrument for self-reporting inhaler technique on any type of device.