A spore-derived mycobiont of a crustose Pyrenula sp. lichen collected in Vietnam was cultivated on a malt-yeast extract medium supplemented with 10% sucrose. Chemical investigation of the cultivated colonies led to the isolation of eight new alkylated decalin-type polyketides (1-8) along with three known compounds. The structures of these compounds were elucidated by spectroscopic and chemical means. This is the first instance of this type of polyketide being isolated from a cultured lichen mycobiont. The isolated polyketides 1 and 7 exhibited inhibitory activities against mammalian DNA polymerases α and β with IC50 values ranging from 8.1 to 19.5 μM. Compound 1 showed cytotoxic effects against the HCT116 human colon carcinoma cultured cell line with an IC50 value of 6.4 ± 0.7 μM.