BACKGROUND: Cancer-related fatigue represents one major cause of reduced quality of life in cancer patients and can seriously affect the physical, emotional, and cognitive functioning impeding coping with the disease. Options for effective treatment of cancer-related fatigue are limited, consisting only of non-pharmacologic interventions like physical activity, psychosocial, and mind-body interventions. Recent evidence suggests that vitamin D3 supplementation might alleviate cancer-related fatigue. However, confirmation in a randomized controlled trial is needed. METHODS: In this multicenter, randomized, double-blind, placebo-controlled trial, 456 colorectal cancer (CRC) patients aged 18 years and older are being recruited in three German rehabilitation clinics. Study inclusion requires hospitalization of at least 3 weeks at such a clinic, a diagnosis of non-metastatic CRC (stage I-III), surgical removal of the tumor within the past 9 months, and season-adapted vitamin D insufficiency or deficiency. Eligible patients are randomly assigned to a personalized regimen of vitamin D3 or placebo for 12 weeks. In the intervention group, a loading dose of 20,000 or 40,000 IU vitamin D3 will be administered daily during the first 11 days, followed by a maintenance dose of 2000 IU daily. Patients will complete questionnaires for secondary outcomes (fatigue subdomains, quality of life and subdomains, depression, functional well-being, and infection frequency). Blood and urine samples will be collected for analyses of safety parameters (hypervitaminosis D, hypercalcemia, hypercalciuria, and renal impairment) and efficacy biomarkers (25-hydroxyvitamin D, HbA1c, white blood cell count, leukocyte subtype counts, serum C-reactive protein, uric acid, creatinine, triglycerides, total, low- and high-density lipoprotein cholesterol). DISCUSSION: This trial tests whether a personalized vitamin D3 dosing regimen reduces or prevents fatigue among non-metastatic CRC patients by treating the underlying vitamin D deficiency/insufficiency. If efficacy can be confirmed, personalized vitamin D3 supplementation could be used as a tertiary prevention measure in addition to non-pharmacological treatments of cancer-related fatigue in CRC patients. We expect to detect an effect of vitamin D3 supplementation on secondary outcomes like quality of life, depression, functional well-being, infections, inflammatory biomarkers, diabetes mellitus, and dyslipidemia. TRIAL REGISTRATION: European Clinical Trials Database: EudraCT-No: 2019-000502-30, January 21, 2019; German Clinical Trials Register (DRKS): DRKS00019907 , April 30, 2019.