Kelp is an underwater plant that generates in most ocean biomes, except frozen, deep frozen, and warm oceans. It can be harvested instantly with any tool and drops a Kelp item when broken. Kelp can be placed underwater on various blocks and naturally converts downward-flowing water into source blocks, making it useful for creating bubble column elevators. It can be smelted into dried Kelp, which can be eaten or crafted into dried Kelp blocks for fuel. Kelp can also be composted with a 30% chance to raise the compost level.

Kelp grows naturally between 2 and 26 blocks tall, increasing in age with each growth stage. Once it reaches an age of 25, it stops growing. Bone meal can be used to accelerate its growth by one block per use. Harvesting the top block resets the age of the Kelp below, allowing for taller-than-normal growth. Kelp farming is similar to sugar cane farming but is easier to automate due to items floating to the surface when harvested.