<p>Hematopoietic stem cell transplantation is a crucial treatment for hematological malignancies. This study aimed to evaluate knowledge, attitudes, and consideration to donate stem cells among first-degree relatives of adult patients with hematological malignancies. This descriptive cross-sectional study included 149 first-degree relatives of patients with hematological malignancies. Data were collected through face-to-face interviews using a 22-item structured questionnaire that assessed demographic characteristics, knowledge levels, and attitudes toward stem cell donation. Regarding general awareness, only 19.5% (<i>n</i> = 29) of participants reported having detailed knowledge about the concept and meaning of stem cell donation, while 9.4% (<i>n</i> = 14) had no idea at all. Regarding the specific technical procedure, 34.9% (<i>n</i> = 52) reported being completely unaware of how it is performed, and only 16.8% (<i>n</i> = 25) had detailed knowledge. Notably, while 88.6% (<i>n</i> = 132) of participants expressed a desire for more information, 62.4% (<i>n</i> = 93) stated they had not previously considered becoming donors. Multivariable logistic regression analysis revealed that knowledge level regarding the concept and meaning of stem cell donation was the only independent factor significantly associated with the prior consideration of donation (<i>p</i> = 0.007). First-degree relatives of patients with hematological malignancies often possess insufficient knowledge and hold misconceptions regarding stem cell donation. Since higher knowledge levels are significantly associated with a greater likelihood of having considered donation, it is essential to implement structured awareness campaigns with simplified, targeted content to expand the donor pool.</p>