Few studies have assessed the convergent validity of the timeline followback for smoking (TLFB-S) among treatment-seeking smokers, and there are no studies assessing its discriminant validity. Further, no studies to date have documented the psychometric properties of the TLFB in the German population. The aim of this study was therefore to test the convergent and discriminant validity of the TLFB-S among adult, treatment-seeking smokers in Germany. In the context of a smoking cessation trial, participants (N=268) completed sociodemographic, TLFB-S and other, self-aggregated, single-item (SA) smoking measures at pretreatment. During the 12-month posttreatment period, participants completed TLFB-S and biochemically validated SA measures of smoking intensity and abstinence at regular intervals. Discriminant validity analyses showed no significant associations for substance-use and sociodemographic variables and measurement discrepancy between TLFB-S and SA measures of pretreatment smoking intensity. Convergent validity analyses indicated that TLFB-S and SA measures of smoking intensity were in high agreement at both pretreatment and posttreatment. Finally, there was high concordance between TLFB-S and SA measures of abstinence at both 1- and 12-month posttreatment. This study replicated and extended the current literature on the TLFB and showed that, even across cultures, it can be a valid measure of various smoking-related variables.