Most crustaceans are known to regenerate their appendages after losing them, and the pattern of regeneration may be related to its function. The pattern of regeneration of the right major cheliped was examined after experimentally induced autotomy and its behavioral function during male-male contests for mates was investigated in the hermit crab Pagurus middendorffii. Males with an autotomized major cheliped regenerated it at the first molt after autotomy and showed smaller growth in body size than in control males. The shape of the regenerated cheliped was more slender than the original, implying that the regenerated major cheliped would be less robust and weaker than previously. The length of the major cheliped of an owner male (guarding a female at the start of a dyadic contest) and the body size of an intruder affected whether or not the contest escalated, but the major cheliped length of the intruder and the body size of the owner determined the outcome of the escalated contest. The intruder used the major cheliped as an offensive weapon to take the female away from the owner by force after the contest escalated. The owner often moved the major cheliped forward, similar to a fencing weapon, before escalation. Such movement of the owner seems to be effective to prevent the intruders from approaching, and the regenerated cheliped of the owner may not require much robustness and strength as a defensive weapon before escalation. Rapid regeneration of the major cheliped in P. middendorffii would therefore be beneficial to minimize the disadvantages incurred by loss of the major cheliped.