According to World Health Organization, over 50,000 hematopoietic stem cell transplants (HSCTs) are performed annually worldwide. Though HSCT can extend life-expectancy and improve disease-related health burdens, it is not without risks. Post-transplant overall survival is improving; therefore, it is imperative that factors contributing to or impeding further improvements are well understood. The purpose of this systematic review is to explore current data on body composition (specifically weight loss, BMI, obesity and sarcopenia) and the relation to HSCT outcomes. A literature search was conducted via PubMed and Web of Science databases. Key words included “body composition,” “sarcopenia,” “hematopoietic stem cell transplant,” “malnutrition,” “body mass index,” and “obesity.” Results indicated that 16 out of 18 analyzed studies found a statistically significant relationship between body composition, in particular higher BMI and weight loss, and at least one survival-related outcome variable (eg., non-relapse mortality, overall survival and/or relapse). Based on the findings of this review, body composition, whether evaluated before or during HSCT, can impact a wide variety of post-transplant outcomes. This speaks to the importance of evaluating patients pre-transplant, identifying potential risk factors for worsened outcomes, and providing immediate interventions in order to optimize transplant outcomes.