In this modern era, the understanding and actualization of zakat (alms-giving) within Muslims have kept developing. One of the developments is the existence of an ijtihad (individual judgment) about professional zakat argued by contemporary ulamā (Muslim scholars). The emergence of this professional zakat concept is included as brand new in the Islamic scientific repertoire and it has ignited controversy among ulamā. In Indonesia, there are two oldest and biggest Islamic organizations, which are Muhammadiyah and Persatuan Islam (Persis). These two organizations have different legal opinion towards professional zakat. This study aims to find out the method of legal reasoning employed by Muhammadiyah and Persis in determining the law of professional zakat. Besides, it aims to indentify the similarities and differences between Muhammadiyah and Persis in perceiving the law of professional zakat. The type of this study was qualitative with analytical and comparative descriptive approach. The data gathering technique was through documentary or bibliography, library research and direct interview. Result shows that the method of legal reasoning used by Muhammadiyah in deciding the law of professional zakat is bayānī (semantic), ta‘līli (rationalistic), istiṣlāhī (philosophic), as well as qīyas. Meanwhile, the method used by Persis in deciding the law of professional zakat is that it prioritizes Al-Qur’an ẓāhir over ta’wīl, and it highlights the bi al-ma’ṡūr interpretation as bayan towards Al-Qur’an. In the perception of Muhammadiyah, professional zakat is compulsory. On the contrary, Persis denies this kind of liability and it gives alternative concept instead, which is professional infak (donation). Nevertheless, both Muhammadiyah and Persis have agreed that the assets revenue excess must be allocated for those who need, either via zakat or infak. Both organizations have also agreed to respect each other’s legal opinion, as the results of ijtihad.