Second language writing instruction has been reconceptualized in accordance with the recent shift of paradigm in second language acquisition from a cognitive to a social perspective. Remarkably, this paradigm shift is in line with the introduction of computer technology and sociocultural theory to second language teaching. To explore the combined effect of computer technology and sociocultural theory on second language writing achievement, the researchers utilized an e-learning platform and activity theory to design an activity theory-based computer-assisted language learning framework appropriate for a second language writing course. For this purpose, 67 sophomores majoring in English translation were selected as the participants of this study. A mixed methods embedded research design was employed using various sources of data collection, including a writing pretest and posttest, two questionnaires, and a semi-structured focus group interview. The results indicated that computer-assisted language learning within the activity theory framework had a significant effect on the writing achievement of the students. Besides, the findings revealed that the students had a positive evaluation of the efficacy of the intervention in their writing achievement. This study has some implications for second language educators, researchers, curriculum developers, and courseware designers.