Over 300 natural disasters affected more than 140 million people in 2014 (Guha-Sapir, Below, and Hoyois 2016). Little is known about the impact of natural disasters on student achievement, an important input into later growth and development. This paper examines the impact of severe flooding on student achievement. By using a difference-in-differences strategy, I exploit the severe flooding in Thailand in 2011 as a natural experiment to measure its effect on the national examination, the O-net. I combine the school-level O-net examination scores from the National Institute of Educational Test Service (NIETS) with school specific data from the Ministry of Education for the academic year 2006-2013. The variation in the impact of the severe flooding allows me to estimate the impact of the flood on the O-net examination scores, a proxy for student academic achievement. The estimates suggest that the flood had a negative and significant effect on all test scores for grade 6, except for social studies. In addition, for grade 9 the flood had a negative and significant effect on all test scores. But, for grade 12 the flood had no significant effect on test scores, except for social studies. The absolute sizes of the change in test score as a result of the flood range from 0.03 to 0.11 standard deviations, depending on the subject and level. My work provides the first empirical evidence of the adverse impact of severe flooding on student achievement. My results suggest that additional support is needed to lower the adverse impact of severe flooding on high-stakes examinations like the O-net examination, because researchers found that a decrease in test scores can lead to lower educational attainment and earnings (Ebenstein, Lavy, and Roth 2016).