This study considered the uncertainty analysis in the measurement of the motion response and added resistance of waves in seafaring conditions. Methods of uncertainty analysis for seakeeping experiments are not well established. Particularly, no previous studies have specifically investigated added resistance measurements in seafaring conditions. In this study, uncertainty analysis based on ITTC Recommended Procedures and Guidelines (2008a), adopted with the method used by the International Organization for Standardization (ISO, 1995), was conducted and the assessment results are summarized in tabular form. The ship model used in the experiment is KVLCC2 ship, which has a hull shape of a typical large tanker. Vertical motion and added resistance were measured at nine regular waves in head sea. For the validation of the measured data, the experimental results were compared with not only those obtained at other test facilities but also the computational results found via strip and Rankine panel methods. In three cases among the nine wave conditions, at least 15 repeated measurements were conducted for the uncertainty analysis of the heave, pitch, and added resistance quantities. In this paper, the sources of uncertainty in added resistance measurement are summarized, and their uncertainties are quantified. From the uncertainty analysis, the sensitivity to each uncertainty source was observed. The present study will be useful in understanding the fundamental uncertainties associated with the towing tank measurement of motion responses and added resistance in waves.