Abstract: While cassava is an important crop in diverse regions of Thailand, little information is available to compare sites, select planting dates, and determine nitrogen (N) requirements. In recent years, the Decision Support System for Agrotechnology Transfer (DSSAT) has been used to develop this information. In order to use DSSAT, the cassava model, namely, CSM-CSCRP-Cassava, needs to be calibrated and validated. A cassava response to nitrogen study was conducted in Thailand during the 2011–2012 growing season. The data were also utilized to calibrate the DSSAT cassava model on cultivar Kasetsart 50. The model could be calibrated to predict the first branching date at 116 days, when it actually occurred at 117 days after planting. The overall average top dry weight and dry root yield were 7.39 and 15.69 t [BULLET OPERATOR] ha−1, which were predicted with a root mean square error of 0.496 and 0.702, respectively. Maximum leaf area index, leaf N (%), and harvested root N (%) were also adequately simulated. Validation experiments were conducted at the diverse Lopburi, Supanburi, and Chonburi sites. Top dry weight and dry root yield were predicted with indexes of agreement of 0.86 and 0.95 in Lopburi, 0.82 and 0.95 in Supanburi, and 0.83 and 0.55 in Chonburi. Nitrogen requirements for maximum yield were overpredicted by the model, indicating additional work is needed to account for negative effects of excessive N. Effects of regional weather conditions and soil types appeared to be adequately predicted by the calibrated model. Improved planting dates were suggested with the calibrated model.