To provide a seal for a dovetail groove for achieving a removal preventing property, ease of assembly, and a sealing performance.

By giving this seal 1 for a dovetail groove has a substantially X-shaped cross section provided with two first lip parts 2a and 2b, having an interval W2 between lip tips that is wider than a groove opening width W1, and two second lip parts 3a and 3b smaller than the first lip parts 2a and 2b, having lip tips and protruded from a dovetail opening; the first lip parts 2a and 2b have the tip interval W2, that is wider than the groove opening width W1, so that twisting of the seal 1 is prevented, to prevent removal of the seal 1. The seal can be installed easily by hooking a recessed part 4a onto a corner 10a of a groove opening, and bending the first lip part 2b, so that ease of assembly can be improved. Since the seal is compressed at sealing to be applied surface-to-surface between the lip tips of the second lip parts 3a and 3b including the recessed part 5, sealing performance can be improved.