Although both scientists and practitioners emphasize the importance of innovative work behavior (IWB) of individual employees for organizational success, the measurement of employees' IWB is still in evolution. Here, we present two multi-source studies that aimed to develop and validate a measure of IWB. Four related dimensions of IWB are distinguished: opportunity exploration, idea generation, championing and application. We derived a tenitem measure of these IWB dimensions from a pilot survey among matched dyads of 81 professionals in a research institute and their supervisors. Next, a survey among a matching sample of 703 knowledge workers and their supervisors from 94 different firms was done. We used confirmatory factor analyses to examine convergent and discriminant validity, and hierarchical multilevel regression to test hypothesized relationships of IWB with participative leadership, external work contacts and innovative output (proposed as an initial nomological network). Results demonstrate strong convergent validity of the IWB measure as all four dimensions contribute to an overall measure of IWB. Support for discriminant validity is weaker as correlations between some dimensions are relatively high. Finally, IWB is positively related with participative leadership, external work contacts and innovative output, providing first evidence for nomological validity.