Polyaniline (PANI), a prominent intrinsically conducting polymer, has gained special attention due to facile synthesis, environmental stability, and reversible doping. However, limitations such as poor processability, limited solubility, electrochemical instability, and weak mechanical properties hinder its full practical potential. Morphology engineering at micro/nano scales has been introduced as a strategy to overcome these limitations. This review addresses recent advances in morphology engineering of PANI nanostructures. First, various nanostructures including nanofibers, nanotubes, nanospheres, hollow microspheres, and hierarchical structures are introduced. Also, synthesis methods of these nanostructures such as chemical, electrochemical, interfacial polymerization, electrospinning, and template or (template-free)-directed growth strategies are described. Next, advanced PANI nanocomposites with metal oxides, carbon nanostructures (graphene, graphene oxide, carbon nanotubes), chalcogens and metal chalcogenides, metal-organic frameworks, and biocompatible polysaccharides are reviewed. The synergistic effects of these components on improving electrical, mechanical, electrochemical, and cycling stability are discussed. Finally, functional applications of these nanostructures in supercapacitors, batteries, gas/bio/electrochemical sensors, smart anti-corrosion coatings, water treatment, photocatalytic degradation, electrochromic materials, electromagnetic interference shielding, and biomedical applications are investigated. Challenges toward commercialization and future perspectives are also mentioned.