An optical fiber is provided, which is unlikely to cause interlayer delamination between a glass optical fiber and a primary coating layer even when it is immersed in water. The optical fiber of the present invention includes a glass optical fiber 1 consisting of a core and a cladding, a primary coating layer 2 overlaid on the glass optical fiber, and a secondary coating layer 3 overlaid on the primary coating layer, wherein the relaxation modulus of the secondary coating layer is set at 400 MPa or less.