A thorough examination of the available literature has revealed a well-established association of obesity and high body mass index (BMI) with an increased risk of various types of cancers, including hematologic malignancies. Specifically, the studies reviewed indicate a clear correlation between obesity and an increased risk of leukemias, lymphomas, multiple myeloma, myelodysplastic syndrome, and myeloproliferative diseases. Despite the established association of obesity and high BMI with hematologic malignancies, the underlying mechanisms remain largely undetermined. The development of hematologic malignancies may be influenced by several mechanisms associated with obesity and high BMI, including chronic inflammation, hormonal imbalances, adiposopathies, and metabolic dysregulation. Furthermore, there is mounting evidence indicating that obesity and high BMI may have a negative impact on the response to treatment and overall survival in patients with hematologic malignancies. This article aims to increase awareness and summarize the current state of research on the impact of obesity on hematologic malignancies, including the mechanisms by which obesity may influence the development and progression of these diseases. In addition, the current review highlights the need for effective weight management strategies in patients with hematologic malignancies to improve outcomes and mitigate the risk of complications.