A woman in her fifties underwent total colonoscopy that showed a 10-mm protruding lesion with a depressed area in the mid-transverse colon (A). The protruding area of the lesion was whitish in color, whereas the depressed area was reddish. Magnifying narrow-band imaging showed thick and branched vessels and expanded crypt openings in the whitish protruding area and irregular disrupted microvessels without surface structure in the reddish depressed area (B, C). The lesion in the transverse colon was predicted to be an invasive cancer accompanied with sessile serrated adenoma and/or polyp (SSA/P) because the magnifying narrow-band imaging pattern of the reddish depressed area was classified as type 3 on the narrow-band imaging classification system (NBI International Colorectal Endoscopic [NICE] Classification), and the pattern of the whitish protruding area was compatible with SSA/P. The patient underwent local transverse colon resection with lymph node dissection. Pathology examination of the resected specimen showed a moderately differentiated adenocarcinoma invading the subserosa, accompanied by SSA/P (D, H&E, orig. mag. x 7.5). Studies have indicated that SSA/Ps can undergo rapid malignant transformation and cause interval cancers. Findings in our patient showed a small and superficial morphology but deep invasion to the subserosa, suggesting such rapid malignant transformation.