Online relationships are increasingly central to many people's lives. As a result, there is a growing need to scientifically examine their psychosocial implications. This study developed and tested the Online Relationship Initiation Scale (ORIS) through classical and item response theory analyses to address this need. An anonymous online survey included 713 adults, aged 18-71 years. The ORIS was tested on psychometric properties and examined for associations with gender and several standardized psychosocial measures. Results demonstrated unidimensionality of nine items, strong factor loadings, and high internal consistency (α = 0.90, ωt = 0.94). All items captured significant information on the latent trait and none showed differential item functioning by sex, age group, or ethnicity. General linear modeling confirmed hypotheses that men were more likely than women to initiate online relationships. Online relationship initiation was not strongly associated with perceived social support, but was positively related to financial distress, and willingness to engage in infidelity or unprotected sex. The ORIS was negatively associated with age and satisfaction with life and showed modest interactions with ethnicity and hours online. This study provided empirical evidence for an interpersonal relationship initiation construct. The ORIS was shown to be a psychometrically sound instrument for evaluating online interpersonal behaviors and their associations with psychosocial and demographic factors. Such psychometrically sound instruments can be useful in exploring online interpersonal behaviors and their significance.