This study aimed to investigate the effect of instruction designed according to the ARCS model enriched with Web 2.0 tools on the mathematics achievement and motivation of secondary school students and to examine student opinions regarding the applied instruction process. A mixed-methods approach was employed in the study since both quantitative and qualitative data were collected. To support quantitative data with qualitative data, a sequential explanatory design was chosen from mixed-methods designs. The research was conducted in the 2022-2023 academic year with the participation of 40 7th grade students enrolled in a public school in Nigde province. The research process was planned for 10 weeks, including a pretest administration in the first week, an eight-week intervention, and a posttest administration in the last week. In the experimental group, a mathematics instruction plan designed according to the ARCS model enriched with Web 2.0 tools was implemented, while in the control group, only the existing mathematics instruction plan was implemented. Quantitative data were collected using the ratio, proportion and percentage achievement tests and the mathematics motivation scale. Qualitative data were collected through focus group interviews with experimental group students. Data analysis was conducted, which included independent samples t-tests, paired samples t-tests, MANOVAs, and content analysis. The findings obtained from this study revealed that teaching with the ARCS model enriched with Web 2.0 tools had a positive effect on students' academic achievement. When the sub-dimensions of motivation were analyzed, no significant difference was found between the groups in the posttest motivation scores, but most of the participant students in the focus group interviews expressed positive opinions about the process and mentioned the effectiveness of the process. This study provides important information about the effectiveness of integrating Web 2.0 tools into the ARCS model in secondary school mathematics education in terms of the effectiveness of the teaching process and its contributions to students' achievement.