Lemnaceae is a botanical name for a family of flowering plants. It is also known as the "duckweed family", as it contains the duckweeds (also known as water lentils).

These plants are very simple, lacking a stem or leaves, but consisting of a small blade-like structure floating on or just under the water surface, with or without simple rootlets. Reproduction is mostly by budding, but occasionally a flower consisting of two stamens and a pistil (some call it an inflorescence with three unisexual flowers) is produced. The fruit is an utricle, a sac containing air and a seed designed to float. The flower of Wolffia is the smallest in the world at 0.3 mm long.

Duckweeds are an important food source for waterfowl and are eaten by humans in parts of Southeast Asia (as khai-nam). Some duckweeds are used in freshwater aquariums and ponds where they may spread rapidly, although in a large pond may be difficult to eradicate once established. The plants can provide nitrate removal (if cropped) and cover for fry. The plants are used as shelter by pond water species, such as bullfrogs and bluegills. The duckweeds are important in the process of bioremediation because they grow rapidly, absorbing excess mineral nutrients, particularly nitrogen and phosphates. A cover of duckweeds will reduce evaporation of water compared to a clear surface.