The red-throated loon is the smallest, slightest of the divers. During the breeding season, the upper body is a solid dark brown. The head and upper neck is grayish, with a large, glossy colored patch on the foreneck. It is white underneath and the tail is dark. In the winter, the face and foreneck are pure white, and the upper part is dark brownish and finely spotted with white.
Males average slightly larger than females, and have a heavier head and bill.
Its neck is thick, and the nostrils are narrow and elongated, as an adaptation to diving. The iris is reddish, especially in adults during the breeding season. The body is designed for swimming, with short, strong legs set far back on the body. The legs are perfect for moving through water, although this design makes walking on land difficult. The three front toes are webbed, and these loons have short, well-defined tails.