An arterial line or arterial catheter is a procedure involving inserting a catheter inside an artery, often the radial but also the ulnar, axillary, brachial, etc. Using an arterial line, one can easily obtain arterial blood gas samples without having to puncture the artery every time, as well as obtain "regular" blood chemistry samples. It also enables invasive arterial blood pressure monitoring (IABP), the continuous monitoring of arterial blood pressure, which is much more accurate than "regular" cuff-based non-invasive blood pressure monitoring.

Arterial lines are very frequently used in critically ill patients in the intensive care unit and elsewhere. Before insertion into radial or ulnar, the Allen test must be performed. The catheter is inserted with the Seldinger technique (over a guidewire).

Medications can not be administered in an arterial line, a central venous or peripheral venous line must be used.