A marlinspike (, sometimes spelled marlin spike, marlinespike, or [archaic] marlingspike) is a tool used in marine ropework. Shaped in the form of a polished metal cone tapered to a rounded or flattened point, it is used in such tasks as unlaying rope for splicing, untying knots, drawing marline tight using a marlinspike hitch, and as a toggle joining ropes under tension in a belaying pin splice.

Marlinspikes are usually about 6–12 inches (15–30 cm) long, but may reach 2 feet (61 cm) or more when used for working heavy cables and ropes. They are usually made from iron or steel, whereas fids, similar in shape and function, are formed from wood or bone. The marlinspike may be a separate tool or one item on a pocket knife.

Sailors who become proficient at knot tying, splicing, and sewing using the marlinspike are said to have mastered marlinespike seamanship, earning them the right to be known as marlin spikes or marlinspike seamen.