Sixteen diterpenes (<b>1</b>-<b>16</b>), along with 10 previously described compounds, including four flavonoids and six diterpenes, were isolated from the aerial parts of <i>Psiadia punctulata</i> growing in Saudi Arabia. The diterpene structures were elucidated using NMR spectroscopy and mass spectrometry data. Furthermore, a DFT/NMR procedure was used to suggest the relative configuration of several compounds. The labdane-derived skeletons, namely, <i>ent</i>-atisane, <i>ent</i>-beyerene, <i>ent</i>-trachylobane, and <i>ent</i>-kaurene, were identified. The extracts, fractions, and pure compounds were then tested against <i>Staphylococcus aureus</i>, <i>Streptococcus mutans</i>, <i>Treponema denticola</i>, and <i>Lactobacillus plantarum</i>. One diterpenoid, namely, psiadin, showed an additive effect with the antiseptic chlorhexidine, with a fractional inhibitory concentration index of less than 1. Additionally, psiadin showed a prospective inhibition activity for bacterial efflux pumps.