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Since its discovery about 50 years ago, HD144941 has generally beenclassified as a peculiar member of the extreme helium (EHe) supergiant stars, avery rare class of low-mass hydrogen-deficient stars. We report the detectionof a strong longitudinal magnetic field based on spectropolarimetry with FORS2on the ESO VLT with surface-averaged longitudinal field strengths as large as-9kG. This is further constrained by the detection of Zeeman splitting ofspectral lines to a field strength of at least 15kG, explaining the recentfinding of surface spots for this star. The quantitative analysis of thestellar atmosphere based on a hybrid non-local thermodynamic equilibriumapproach and new optical spectra yields an effective temperature of22000$\pm$500K, a logarithmic surface gravity of 4.20$\pm$0.10, and a surfacehelium fraction of 0.950$\pm$0.002 by number. While the metal abundances areabout a factor of 10 sub-solar in absolute number, the metal-to-hydrogen ratiosare typical of massive early-type stars, indicating that helium fallback in aweak, fractionated stellar wind in the presence of a magnetic field took place-- the canonical mechanism for the formation of the helium-strong phenomenon.Both the spectroscopic and the Gaia EDR3 parallax imply HD144941 to be aluminous massive star. Kinematically, we argue that HD144941 has reached itshigh Galactic latitude as a runaway star. We conclude that instead of being acomparatively high-gravity low-mass EHe star, HD144941 is by far the mostextreme member of the magnetic massive helium-strong stars, with almost allatmospheric hydrogen substituted by helium.

HD144941: The most extreme helium-strong star