The present study investigates the relationship between L2 metalinguistic knowledge and L2 achievement among intermediate-level adult Turkish EFL learners studying at a large scale Turkish university. Metalinguistic knowledge refers to the ability to correct a grammatically incorrect structure in English and explain why it is incorrect, and identify and explicitly state the grammatical role of parts of speech in L2 sentences. It is assessed using the two-section Metalinguistic Knowledge Test (MKT) designed by the researcher. L2 achievement, on the other hand, refers to the participants?L2 knowledge at a specific level and is operationalized as the ability to repeat language elements that have been taught and mastered. It is assessed using the mid-term exam that consists of grammar, vocabulary, listening, reading, speaking and writing sub-tests. Results of the MKT indicate that L2 metalinguistic knowledge is weak among intermediate-level adult Turkish EFL learners. In addition to this, a correlation analysis and a series of bivariate and multiple regression analyses reveal that L2 metalinguistic knowledge significantly contributes to L2 writing achievement explaining 19.9% of the variance in participants? writing exam scores. The findings of the present study are discussed within the light of the previous research. Additionally, considering that metalinguistic knowledge benefits second language acquisition (SLA), some implications are suggested accordingly.