Introduction: Hypokalaemia or potassium deficiency in cancer patients is the second highest Electrolyte Disorder (ED) after Hyponatremia. Deficiency of potassium can cause sudden vital organ failure in malignancy, if it is not diagnosed and manage immediately. Additionally, there remain many risk factors related to malignancy and its treatment to developing potassium deficiency. This narrative review was conducted to document an overview of aetiology of Hypokalaemia in patients with different stage of various types of cancer. Aim: The narrative review could be informative and important in recognizing Hypokalaemia in oncology treatment and management. Research Question: What is the aetiology of developing hypokalaemia in cancer patients? Methodology: Literatures were selected for review based on inclusion and exclusion criteria. Selection was done purposively based on several outcome and information regarding potassium deficiency in cancer patients. Articles were reviewed by mainly considering the population and results of the study. Main author reviewed the papers and cross-checked with third author, and finally came to conclusion after consensus. Inclusion Criteria: a) Study on cancer patients developing hypokalaemia, b) Study on hypokalaemia as an outcome of cancer treatment and management, c) Study on mechanism of developing hypokalaemia in cancer patients, and d) Study on hypokalaemia resulting from risk factors as a consequence of cancer. Exclusion Criteria: a) Study on hypokalaemia not due to cancer treatment or management, b) Incomplete study, c) Not found in online publications/digital libraries/e-journals. Study Period: Review period was January to March 2021. Literature Search: Keyword based scientific paper search underwent through Google Scholar, PubMed and Embase. Results and Discussion: Aitiology of hypokalaemia can be grouped under four headings: a) Malignancy, itself, b) treatment of the cancer that includes chemotherapy and targeted therapies; c) concomitant drugs and d) concomitant diseases; endocrine dysfunction, toxic epidermal necrolysis, inflammatory bowel diseases among others. There are three mechanisms found to be involved which are caused by the above mentioned risk factors. They are: an inadequate potassium intake, redistribution of potassium among intra- and extracellular compartments, and potassium losses. Conclusion: Hypokalaemia in cancer patients is a life-threatening situation and increases mortality and morbidity. Recognizing and understanding the aetiology could be helpful for the clinicians, especially the oncologists to reduce the suffering and improve palliative management in cancer patients.