Respiratory epithelial adenomatoid hamartoma (REAH) is a benign lesion of the nasal cavity and paranasal sinuses. Here, we report the clinicopathological characteristics of REAH identified in 2065 cases with nasal/paranasal polypoid lesions treated with endoscopic sinus surgery (ESS) at our hospital from 2008 to 2021. Cases including the olfactory area were reviewed and 50 patients of REAH were identified pathologically (50/2065, 2.4%). The average age was 58.9 years old and the male/female ratio was 45/5. Grossly, REAH showed a whitish surface and elastic firm consistency. The histopathological characteristics included proliferation of small to medium-sized glands composed of ciliated respiratory epithelium containing goblet cells; thickening of the basement membrane compared to that for inverted papilloma (9.6 ± 2.4 vs. 1.3 ± 1.6 µm, p < 0.001); and no intra-epithelial neutrophilic infiltration. Among the REAH cases, 81% were associated with sinonasal inflammatory polyps. Many olfactory cleft polyps were REAH (38/98, 39%). The rate of REAH found in ESS in the last 7 years was higher than that in the first 7 years (3.17% vs. 1.62%, p = 0.032). Our results in Japanese patients are similar to those found in other countries, including male predominance. REAH is relatively common and that 39% of polyps taken from olfactory clefts are REAH.