Eight new sulfur-bridged pyranonaphthoquinone (PNQ) dimers, naquihexcins C-J (<b>1</b>-<b>8</b>), a new PNQ monomer, naquihexcin K (<b>10</b>), and three known analogues (<b>9</b>, <b>11</b>, and <b>12</b>) were isolated from <i>Streptomyces</i> sp. KIB3133. The new structures were elucidated by interpretation of spectroscopic data. Dimer <b>4</b> was synthesized via a cascade S<sub>N</sub>2 reactions between two monomers and sodium sulfide, an approach motivated by the proposed biosynthetic pathway of dimeric pyranonaphthoquinones. Naquihexcin E (<b>3</b>) exhibited moderate HIV-1 inhibitory activity. Naquihexcins C (<b>1</b>), E (<b>3</b>), and I (<b>7</b>) showed inhibitory effects against two tumor cell lines (HL-60 and MCF-7) with IC<sub>50</sub> values ranging from 1.4 to 16.1 μM.