This study investigates the validity of syntactic priming as a measure of implicit language aptitude. Syntactic priming refers to the tendency to reproduce a linguistic structure due to a previous exposure to the structure. The validity of the construct was verified by collating evidence for divergent validity—whether it is dissociable from explicit aptitude; convergent validity—whether it is correlated with other measures of implicit aptitude; and predictive validity—whether it is predictive of learning attainment. One hundred sixty-six university EFL learners completed three tests of implicit aptitude: syntactic priming, sequence learning, and LLAMA_D; three tests of explicit aptitude: LLAMA_B, _E, and _F; and three tests of L2 proficiency: untimed grammaticality judgment, metalinguistic knowledge, and elicited imitation. The results showed that syntactic priming was dissociable from explicit aptitude, but it failed to converge with the other measures of implicit aptitude, and it also failed to predict L2 proficiency. The results also showed that priming was negatively correlated with sequence learning and that sequence learning was a negative predictor of learners’ metalinguistic knowledge. On the other hand, the construct validity of explicit aptitude was strong. The results suggest the multidimensionality of implicit aptitude and the need for more research into the construct validity of syntactic priming as a cognitive ability for implicit learning.