An accurate re‐evaluation of the instrumental magnitude has been performed for the 28 December 1908 Messina Straits earthquake (Southern Italy), one of the most destructive events ever recorded in the Mediterranean region. Despite the crucial importance of this earthquake for seismic hazard assessment, magnitude values reported in the literature show a wide range of variability, reflecting the inherent uncertainties typical of early instrumental data. This variability suggests the need of a rigorous revision of data and methods used for magnitude estimation. With this purpose, we (1) conducted a comprehensive collection and critical analysis of analog seismograms and station bulletins available for the 1908 event, and (2) rigorously applied the most recently revised approaches for determining body‐ and surface‐wave magnitudes of historical events, also following the recommendations of the International Association of Seismology and Physics of the Earth’s Interior. We present the re‐estimation of the body‐wave magnitude "mB" using the original Gutenberg–Richter formula, and the surface‐wave magnitude "Ms" employing both the "Moscow–Prague" formula and the approach proposed by Abe and Noguchi in 1983. Moreover, this study represents the first instance in which data from undamped instruments have been incorporated into reassessment of this magnitude. The results of our analysis suggest a more reliable final magnitude range between 6.9 and 7.0 for the 1908 earthquake and also emphasize the importance of using consistent and rigorous methodologies in historical seismic research for contributing to more accurate assessments of seismic hazard evaluations.