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Background: Chronic inflammation is implicated in the development of colorectal cancer (CRC) and its precursor; colorectal adenomatous polyps (CAP). Some dietary factors are important triggers for systemic inflammation. Therefore, the present study aimed to investigate the association between the dietary inflammatory index (DII®) and the risk of CRC and CAP in an Iranian case-control study. Methods: 134 newly diagnosed CRC patients, 130 newly diagnosed CAP patients, and 240 hospitalized controls were recruited using convenience sampling. Energy-adjusted DII (E-DII) scores were computed based on dietary intake assessed using a reproducible and valid 148-item food frequency questionnaire. Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (CI) after adjusting for confounders. Results: The E-DII score ranged between −4.23 (the most anti-inflammatory score) to +3.89 (the most pro-inflammatory score). The multivariable-adjusted ORs for participants in the 3rd tertile compared to the 1st tertile was 5.08 (95%CI: 2.70–9.56; P-trend < 0.0001) for CRC and 2.33 (95% CI: 1.30–4.02; P-trend = 0.005) for CAP. Conclusions: Our findings suggest that more pro-inflammatory diets, indicated by higher E-DII scores, might increase the risk of both CRC and CAP. Future steps should include testing these associations in a prospective setting in Iran.