Languaging--that is, using language to mediate thinking, orally or in writing--facilitates second language (L2) learning. As languaging requires the deliberate, analytical action of L2 learners, learners with higher language aptitude are likely to benefit more from languaging than those with lower aptitude. Thus, this study investigated the extent to which language aptitude mediates the effects of written languaging (WL) on L2 learning. Pre-intermediate Japanese university students were assigned to +WL or -WL groups. The +WL group was instructed to compare their composition with the model text and explain, in writing, the use of articles (i.e., WL). In contrast, the -WL group copied the model 3 times. Examination of the association between the scores on aptitude tests (i.e., LLAMA, Language Aptitude Battery for Japanese, and Modern Language Aptitude Test) and the gain scores on assessment tests (pre-, post-, and delayed posttests) revealed that half of the correlations were significant for the 2 test types in the -WL group. However, no such correlations were found in the +WL group. Thus, WL might have benefitted the +WL participants by facilitating their noticing and reflecting on the linguistic issues, regardless of their aptitude profiles. We argue that WL may be a more influential factor than aptitude.