Seaweed oil or seaweed oil is an alternative to liquid fossil fuels that uses seaweed as its source of energy-rich oils. Like fossil fuel, seaweed fuel releases when burnt, but unlike fossil fuel, algae fuel and other biofuels only released recently removed from the atmosphere via photosynthesis as the seaweed or plant grew. Seaweed does not require fresh water and can be grown in the sea.

Seaweed can be converted into various types of fuels, depending on the technique and the part of the cells used. Also, using such techniques as fast hydrothermal liquefaction, oil yield for the seaweed can be increased.

Transesterification of seaweed oil (into biodiesel) is possible with such species as Chaetomorpha linum, Ulva lactuca, Enteromorpha compressa (Ulva).