The present study aimed to design and evaluate the psychometric properties of the Paternal Adaptation Questionnaire (PAQ). The study was a mixed (qualitative and quantitative) sequential exploratory study. In the qualitative phase, a preliminary questionnaire with 210 items emerged from in-depth interviews with 17 fathers and 15 key informants. In the quantitative phase, psychometric properties of the PAQ were assessed. Considering cutoff points as 1.5 for item impact, 0.49 for content validity ratio (CVR), and 0.7 for content validity index (CVI), items of the questionnaire were reduced from 210 to 132. Assessment of the content validity of the questionnaire demonstrated S-CVR = 0.68 and S-CVI = 0.92. Exploratory factor analysis resulted in the development of a PAQ with 38 items classified under five factors (ability in performing the roles and responsibilities; perceiving the parental development; stabilization in paternal position; spiritual stability and internal satisfaction; and challenges and concerns), which explained 52.19% of cumulative variance. Measurement of internal consistency reported a Cronbach’s α of .89 for PAQ (.61-.86 for subscales), and stability assessment of the PAQ through the test–retest demonstrated Spearman’s correlation coefficients and intraclass correlation coefficient of .96 (.81-.97 for subscales). It was identified that the PAQ is a valid and reliable instrument that could be used to assess fatherhood adaptation with the paternal roles and fathers’ needs, as well as to design appropriate interventions and to evaluate their effectiveness.