This paper is a report on the study of the relationship between foreign/second language (L2) motivation and vocabulary knowledge. The population of the study is the cadets of a military academy in Saudi Arabia who study English as a foreign language. The total number of participants is 195 Saudis. The author uses the L2 Motivational Self System to investigate the cadets' L2 motivation and the vocabulary test "X-Lex" (Meara & Milton, 2003) to measure their vocabulary size (VS). The correlation analysis shows a positive moderate statistical correlation between the students' L2 motivation and their VS because the Ideal L2 Self, the Language Learning Attitudes, and the Intended Learning Effort had a linear relationship with the VS. Nevertheless, the regression analysis reveals that none of these motivational scales is a predictor of the students' VS. Finally, the correlation analysis and the regression analysis disclose a statistically significant correlation between the Saudi cadets' self-perception as competent English users and their L2 motivation as well as their VS.