[3] The reserve is mostly flat, consisting mainly of peat swamp forest, with a small area of mangroves.

[5] In 2016 large fires in peat bogs, which spread to Klias and other forest reserves from nearby open burning, contributed to the 2016 Malaysian haze.

[7] Resident tree species include Dryobalanops rappa, Madhuca motleyana, Shorea platycarpa, Dactylocladus stenostachys and Gonystylus bancanus.

[5] Klias Forest Reserve is home to threatened mammals including banded palm civet, pig-tailed macaque, proboscis monkey, sambar deer, Bornean bearded pig, mousedeer and the critically endangered Sunda pangolin.

[5] Bird life in the reserve includes the hook-billed bulbul, whose Sabah range is confined to the Klias Peninsula, and the rare species grey-breasted babbler and scarlet-breasted flowerpecker.