Recent investigations have suggested that both metacognition and emotions play important roles in second language (L2) learning. However, the majority of these studies focused on negative emotions, such as anxiety, or considered high school or university students, neglecting primary school age students, which is frequently the starting period in which a L2 is learned. Moreover, none of these studies investigated both metacognition and emotions in relation to L2 learning. Thus, the present study aimed to explore within a short-term longitudinal design the role of both metacognition and emotions in learning English as a L2 starting from an early stage, namely, in primary school. We included 305 Italian 2nd and 3rd graders (Mage = 7.44 years; SD = 0.59; 48.5% females). They were evaluated twice, at the beginning (T0) and the end of the school year (T1), on an English vocabulary test, metacognitive monitoring and control of performance on the English vocabulary test, and achievement emotions (i.e., enjoyment, anxiety, and boredom) related to learning English as a L2. Basic cognitive abilities were also assessed at T0. The results showed that both metacognitive monitoring and control abilities significantly predicted children's English vocabulary improvements from the first to the second test administration points. Among the achievement emotions, only boredom was a strong predictor of English vocabulary learning. These outcomes showed that metacognitive and emotional variables are relevant beginning at the early stage of L2 learning.