Background: Processed foods have been implicated in the pathogenesis of inflammatory bowel diseases (IBD). Our goal was to develop a validated processed foods frequency questionnaire (PFQ) and assess its reliability and validity. Methods: We recruited adult IBD patients to fill-in a PFQ in this prospective single-center study. Food intake was categorized into three groups of processed food levels: unprocessed, processed, and ultra-processed. Reliability was assessed by comparing the PFQ results of each patient at 2 time points. Validity was assessed by comparing the PFQ results to a 3–7 day food diary (FD), and by comparing urine sodium as a biomarker for the high intake of sodium that is mostly present in processed food. Results: Eighty-six IBD patients were enrolled. Good test–retest reliability was indicated by intraclass correlation of 0.75–0.88 for the different food processing levels. Validity was fair-to-strong as assessed by correlations for different levels of processed food intake between FDs and PFQ, ranging between 0.43 and 0.64 (Pearsonr, P < 0.001), and further supported by higher mean urine sodium levels in patients with high processed foods consumption compared with low consumption (104.57 ± 53.26 vs. 78.62 ± 39.08 mmol/L, respectively, P = 0.022). Agreement between PFQ and the FD in categorizing patients to high and low processed food consumption groups was fair (Kappa 0.23–0.35). Conclusions: The PFQ is a reliable and valid tool for the assessment of processed foods consumption in IBD patients and can be utilized for studying the association between processed food consumption and IBD etiopathogenesis.