Xi1 Canis Majoris, Latinized from ξ1 Canis Majoris, is a Beta Cephei variable star in the constellation Canis Major. It is approximately 1,400 light years from Earth. ξ1 Canis Majoris is a blue-white B-type star. It has generally been assigned a luminosity class of III (giant) or IV (subgiant), for example B1III or B0.5IV. Comparison of its properties with model evolutionary tracks suggest that it is a main sequence star about three quarters of the way through its main sequence lifetime. The apparent magnitude varies from +4.33 to +4.36 with a period of 5.03 hours. Its pulsations cause its radius to vary by 1.0% to 1.5%. At the same time its effective temperature by about 500 K above and below its mean temperature. ξ1 Canis Majoris has the longest known rotation period of any B class star, taking around 30 years to complete one revolution on its axis. This is thought to be due to magnetic braking; ξ1 Canis Majoris has the strongest magnetic field of any β Cephei star and would be expected to spin down completely in around four million years. It also has the strongest and hardest X-ray emission of any β Cephei star. ξ1 Canis Majoris forms a naked eye pairing with ξ2 Canis Majoris a little less than a degree away. The Washington Double Star Catalog lists two 14th magnitude companions about 27 " away. In addition, an unseen close companion is suspected due to some faint emission lines in the spectrum that are best explained by a Be star invisible against the brighter primary.