The purpose of this study is to explore the topics, sub-elements, and characteristic expressions that constitute feedback content by analyzing teacher feedback contents on asynchronous online discussion activities. Teacher feedback data consisting of 421 sentences provided to a total of 105 students (73 males, 32 females) were collected from online discussion activities conducted in four core liberal arts classes at one university in the second semester of 2021. The collected data were organized using an Excel program, and the data were analyzed using a mixed methods approach applying a text mining technique for quantitative analysis and a content analysis method for qualitative analysis. The study results are as follows. Four themes of teacher feedback were extracted according to the text mining technique: (a) discussion participation and exchange of opinions, (b) evidence for arguments, (c) methods of presenting and citing evidence, and (d) selection of positions and argumentation. As a result of the analysis according to the content analysis method, four intermediate categories (understanding, participation, expression, and evidence) and 11 subcategories were found in the content category, and three intermediate categories (motivation, recognition, and correction) and five subcategories were found in the role category. More specifically, the content of the teacher feedback included praise and confirmation of the process of forming opinions according to an understanding of the topic, recognition and encouragement of participation in discussions, correction of how to explain and present opinions, and review and explanation of evidence collection and use. Based on these research results, implications for teacher feedback on asynchronous online discussion activities are highlighted.