The accessibility of learning media is one of the determining factor of student skills in dealing with the 21 st -century demands. This study aimed at developing the onlinediscussion-forum-based Gen-21cs application. This Design Development Research (DDR) used the Richey and Klein Model which comprised of six phases, namely, 1) identifying the problem by determining learning indicators and supporting literature, 2) determining the purpose of development, 3) making the design and development of tools 4 ) approve device testing, 5) try device test results and 6) communicate device test results. The data gained were analyzed using descriptive qualitative. The development results showed that the Gen-21cs application can be used in learning process and facilitated students to conduct online discussion. Furthermore, the validation results indicated that the media was feasible to use with 'very good' category.