Background: In preoperative blood management of colorectal cancer patients, intravenous iron therapy is increasingly used to treat anaemia and prevent red blood cell transfusions. However, while iron deficiency is the most common cause of anaemia, little is known about the prevalence and namely type of iron deficiency in this population, whereas both types of iron deficiency (i.e. absolute and functional iron deficiency) are recommended to be treated differently by international cancer guidelines. Objective: The aim of present study is to investigate the prevalence and namely type of iron deficiency in colorectal cancer patients, and to assess its clinical relevance. Methods: Preoperative iron status, clinical parameters (i.e. age, ASA classification, tumour location, tumour stage) and postoperative complications were retrospectively collected for all newly diagnosed colorectal cancer patients in our institution over a 3-year period. Results: Iron deficiency was observed in 163 (48.1%) of 339 patients. Of these iron-deficient patients, 3.7% had an isolated absolute iron deficiency (AID) and 15.3% a functional iron deficiency (FID), while the rest had a combination of AID and FID. Anaemia was present in 66.1% of iron-deficient patients. Iron deficiency was significantly associated with an increased postoperative complication rate (univariable OR 1.94, p = 0.03, multivariable OR 1.84, p = 0.07), with right-sided tumours (p < 0.001), high ASA classification (p = 0.002), advanced tumour stage (p = 0.01) and advanced age (p = 0.04). In comparing clinical parameters between patients with AID and FID, advanced age was significantly associated with FID (p = 0.03), and the presence of anaemia with AID (p = 0.02). Conclusion: In preoperative colorectal cancer patients, there is a high prevalence of iron deficiency, including a high percentage of patients with—a component of—functional iron deficiency, associated with the increased postoperative complication rate. As both types of iron deficiency require a different treatment strategy, our results illustrate the therapeutic potential of especially intravenous iron supplementation in patients with severe iron deficiency and stress the urgency of routinely monitoring preoperative iron status and differentiation between types of iron deficiency. As iron therapy may also be potentially harmful in respect to stimulation of tumour growth, future clinical trials assessing the long-term effect of iron therapy are necessary.