In order to find new bioactive compounds from the genus Leccinum, a further chemical investigation of Leccinum extremiorientale was carried out. One new pyrrole alkaloid, 2-[2-formyl-5-(methoxymethyl)-1H-pyrrol-1-yl]acetic acid (1), together with two known ones, 4-[2-formyl-5-(methoxymethyl)-1H-pyrrol-1-yl]butanoic acid (2) and 4-[2-formyl-5-(hydroxymethyl)-1H-pyrrol-1-yl]butanoic acid (3), were isolated from the ethyl acetate extract of the fruiting bodies of L. extremiorientale for the first time. Their structures were identified on the basis of 1D and 2D NMR spectroscopy as well as MS. Compounds 1–3 were tested for cytotoxicity against K562, BEL7702, and SGC7901 human cancer cell lines, but showed poor activity with IC50 values greater than 40 M.