Objective: The Internet offers many potential benefits to conducting smoking and other health behavior research with young adults. Questions, however, remain regarding the psychometric properties of online self-reported smoking behaviors. The purpose of this study was to examine the reliability and validity of self-reported smoking and smoking-related cognitions obtained from an online survey. Methods: Young adults (N = 248) age 18 to 25 who had smoked at least 1 cigarette in the past 30 days were recruited online and completed a survey of tobacco and other substance use. Results: Measures of smoking behavior (quantity and frequency) and smoking-related expectancies demonstrated high internal consistency reliability. Measures of smoking behavior and smoking stage of change demonstrated strong concurrent criterion and divergent validity. Results for convergent validity varied by specific constructs measured. Estimates of smoking quantity, but not frequency, were comparable to those obtained from a nationally representative household interview among young adults. Conclusions: These findings generally support the reliability and validity of online surveys of young adult smokers. Identified limitations may reflect issues specific to the measures rather than the online data collection methodology. Strategies to maximize the psychometric properties of online surveys with young adult smokers are discussed.