Aims: Breast cancer is the most common cancer in women worldwide. Breast cancer survivors often experience arm and shoulder pain, limited shoulder range of motion, and lymphedema as the most common post-treatment morbidities. All these morbidities can be considered as the main causes of the fear of movement, called kinesiophobia. This is the first study aims to evaluate the biological and psychological causes of kinesiophobia in breast cancer-related lymphedema (BCRL), the relationship between kinesiophobia and BCRL, and the impact of kinesiophobia on patients’ upper extremity function and quality of life. The biological and psychological causes of kinesiophobia in women with breast cancer-related lymphoedema were investigated for the first time in the literature. Methods: Patients with BCRL were included in the study. Demographic and clinical information including age, educational status, body-mass index (BMI), and dominant upper extremity were recorded. BCRL stage (International Society of Lymphology (ISL) Scale), Quality of Life Scale [European Organisation for Research and Treatment of Cancer Quality of Life (EORTC QLO-C30)], upper extremity functional status [Quick-Disabilities of the Arm, Shoulder and Hand Score (Quick-DASH)], Tampa Kinesiophobia Scale (TKS), Kinesiophobia Causes Scale (KCS) were assessed. Results: The mean age of the 114 patients included in the study was 58.25±9.41 years. A total of 100 patients exhibited a TKS score above 37, indicative of kinesiophobia. There was a statistically significant positive correlation between age and BMI and total TKS score (p<0.05). The TKS score (46.18±6.61) was significantly higher in 66 patients with a dominant limb affected by BCRL (p<0.05). No significant correlation was found between the lymphedema stage (ISL) and quick-DASH (p>0.05). However, the relationship between the Quick-DASH score and the TKS score was significant (p<0.05). A strong significant positive correlation was observed between the TKS score and the KCS score (p=0.0001). Conclusion: In our study, the severity of kinesiophobia was higher in patients with more limited upper limb function. Psychological (self-acceptance, self-assessment of motor predispositions, body care) and biological causes (morphological, individual need for stimulation, energetic substrates, power of biological drivers) increased the severity of kinesiophobia. Biological causes were found to cause more kinesiophobia and affect upper limb function in MKBL. In particular, impairment in the strength of biological impulses was found to be one of the main causes of kinesiophobia. Understanding the causes of kinesiophobia in MDL may improve rehabilitation programs and lead to the development of new strategies to help patients support treatment to reduce fear of movement.