Background and objective: We aimed to validate the diagnostic accuracy of appendicitis, its severity, its description, and the surgical approach, including open or laparoscopic appendectomy and diagnostic laparoscopy, in the Danish National Patient Registry (DNPR) against information from the electronic medical records. Methods: A random sample of 1046 patients of all ages and sexes recorded in the DNPR from the Capital Region of Denmark during 2010–2015 was investigated. Patients’ admission had to include a discharge code for appendicitis (K35−K379) according to the International Classification of Disease version 10 (ICD-10) alone or in combination with a surgical code for appendectomy or the surgical code for a diagnostic laparoscopy. We calculated the positive predictive values (PPVs) with 95% confidence intervals. Results: Data from a total of 1018 patients were available for data analysis. The ICD-10 codes for appendicitis resulted in a good PPV of >95% when combined with a surgical code. ICD-10 codes combined with the surgical codes could discriminate between uncomplicated and complicated appendicitis with some caution as the PPVs ranged from 68% to 93%. Only the surgical code for laparoscopic appendectomy yielded a good PPV (99%) for the actual surgical approach. The surgical code for a diagnostic laparoscopy alone did not cover a normal diagnostic laparoscopy. Conclusions: The diagnostic accuracy of appendicitis and appendectomy in routinely collected administrative health data is good for ICD-10 codes for appendicitis in combination with a surgical code and especially for laparoscopic appendectomy. Uncomplicated and complicated appendicitis can be distinguished with some caution.