Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by the death of both upper and lower motor neurons in the brain, brainstem, and spinal cord. In approximately 95% of cases, ALS is associated with the nuclear-cytoplasmic mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43). The diagnosis of ALS is based solely on clinical assessments, including neurological examinations and electromyography studies, and currently, there is no reliable biomarker for diagnosing ALS using antemortem tissues. Additionally, while TDP-43 positron emission tomography ligands are being investigated, they are not yet widely available for assessing brain TDP-43 pathology. Therefore, a robust fluid biomarker that reflects pathological TDP-43 accumulation in the central nervous system is essential for confirming an ALS diagnosis. In this context, we provide a comprehensive summary of the current status of fluid biomarker development, focusing on TDP-43 pathology, and discuss the existing limitations as well as future directions for ALS biomarker discovery.