Justicia adhatoda L. is an important medicinal plant belonging to the family Acanthaceae and is widely used in Ayurveda, Unani, Siddha, and folk medicine for the treatment of various diseases. The present systematic review compiles ethnomedicinal uses and phytochemical constituents reported from different plant parts including roots, leaves, flowers, fruits, and whole plant preparations. Information was collected from research articles, review papers, books, and scientific databases such as Google Scholar, ScienceDirect, PubMed, Wiley Online Library, PubChem, and NISCAIR Online Periodicals Repository (NOPR). A total of 39 published articles were critically analyzed for this review. Ethnomedicinal studies revealed that different plant parts are extensively used for the treatment of respiratory disorders such as cough, asthma, bronchitis, cold, tuberculosis, and tonsillitis. Leaves were identified as the most frequently used plant part and were traditionally administered in the form of extracts, decoctions, juice, paste, and ash for treating fever, diabetes, rheumatism, skin diseases, and respiratory ailments. Phytochemical investigations documented 128 bioactive compounds belonging to alkaloids, flavonoids, terpenoids, steroids, chalcones, amino acids, glucosides, essential oils, and other compounds. The rich ethnomedicinal importance and phytochemical diversity of Justicia adhatoda support its traditional therapeutic applications and highlight its potential for future pharmaceutical and herbal drug development.