This research discusses the methodology of Imam al-Nawawi in formulating the concept of weak hadith and his approach in using it as dalil syar’i. Something needs to be clarified when al-Nawawi (d. 676 H) accepted the usage of hadith dha’if as an authoritative legal argument despite being part of the leading hadith experts who continually protect the authenticity of Prophet's Sunnah. This study addresses two main issues: How does al-Nawawi describe a hadith as dha’if, and how does he use dha’if hadith as a basis for legal rulings? The data analysis method used in this research is descriptive qualitative analysis. The researcher adopts a historical-philosophical approach for this study. This study reached two main conclusions: firstly, the definition of hadith dha’if proposed by an-Nawawi does not contradict the description of the majority of hadith scholars. However, an-Nawawi added some special provisions that must be considered, so there is no rush to weaken the hadith and ignore it to be practiced or make it a legal argument. He used hadith dha’if as supporting evidence or argument in specific religious matters under certain conditions. Secondly, the information leads us to conclude that an-Nawawi's social and scientific background played an important role in shaping his thinking and applying his religious methodology. It also shows that an-Nawawi paid close attention to the continuity of the sanad from when he acquired knowledge to when he passed it on to others. An-Nawawi's concern for sanad indicates its importance as everything establishes the religion's authenticity. Even if the quality of the sanad is weak, it is not a problem to be used as long as the weakness is explained and supported with other authentic sources. If it is not found, we can use other ijtihadi arguments, such as qiyas.