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We consider a decaying magnetic dark matter explaining the X-ray line at 3.55keV shown recently from XMM-Newton observations. We introduce two singletMajorana fermions that have almost degenerate masses and fermion-portalcouplings with a charged scalar of weak scale mass. In our model, anapproximate Z_2 symmetry gives rise to a tiny transition magnetic momentbetween the Majorana fermions at one loop. The heavier Majorana fermion becomesa thermal dark matter due to the sizable fermion-portal coupling to the SMcharged fermions. We find the parameter space for the masses of dark matter andcharged scalar and their couplings, being consistent with both the relicdensity and the X-ray line. Various phenomenological constraints on the modelare also discussed.

Magnetic dark matter for the X-ray line at 3.55 keV