The ubiquitin-proteasome system (UPS) is a critical cellular mechanism that ensures protein quality control by selectively degrading damaged or unnecessary proteins. This process is essential for maintaining cellular balance and preventing the accumulation of harmful protein aggregates. However, when the UPS becomes dysfunctional, it can lead to several diseases. In neurodegenerative disorders such as Parkinson’s, Alzheimer’s, and Huntington’s disease, faults in the UPS lead to the buildup of toxic proteins such as α-synuclein, amyloid-beta, and huntingtin. These aggregates disrupt normal neuronal activity, increase oxidative stress, and promote neuroinflammation, ultimately causing progressive neurodegeneration. Similarly, in cancer, the UPS can malfunction in ways that either accelerate the degradation of tumor-suppressing proteins like p53 or prevent the breakdown of proteins that drive unchecked cell division, leading to tumor growth. Beyond neurodegeneration and cancer, UPS dysfunction also impacts the immune system by affecting antigen processing and inflammatory responses. This can allow cancer cells to evade immune detection or contribute to autoimmune diseases and chronic inflammatory conditions. Additionally, cardiovascular diseases, metabolic disorders, and infectious diseases have all been linked to UPS irregularities, further demonstrating its broad impact on human health. Targeting the UPS for therapeutic intervention has gained significant interest. Proteasome inhibitors and drugs that modulate ubiquitin ligases are being explored as potential treatments to restore protein balance and slow disease progression. A deeper understanding of how the UPS operates in different diseases could lead to more effective therapies and improved patient outcomes.