The purpose of this study was to design and develop a theory-based summary writing online tool, named Summary Writing-Pal (SW-PAL), to assist English as a second language students in improving their summary writing. It also evaluates the effectiveness of SW-PAL in enhancing the students' summary writing performance and examines their perceptions about it. This mixed-method empirical study involved 53 English as a second language students majoring in computer science at a local university. Two intact groups were randomly chosen as the control and experimental groups with 26 and 27 students, respectively. The control group was taught using the conventional method, while the experimental group was taught using SW-PAL. Both groups were given a pre- and post-summary writing test. A Split-Plot Analysis of Covariance test was used to examine the effectiveness of the SW-PAL tool. A focus group interview was conducted to gather qualitative data on perceptions about the SW-PAL tool. Quantitative findings revealed that students' summary writing performance improved significantly due to the SW-PAL with a large effect size of 0.42. Qualitativewise, the users perceived SW-PAL to be useful as a motivating, challenging, and self-learning tool. Recommendations for practice for language instructors who wish to incorporate such a tool into their language instruction and suggestions for future research are discussed.