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We obtained a robust regional body wave magnitude scale mb(Pn) for earthquakes along the Northern mid‐Atlantic Ridge by using Pn wave recorded at continental stations. The new magnitude scale is mb(Pn) = log10 A [nm] − 1.86 log10 (100/Δ) [km] + C + 1.62, where A is the zero‐to‐peak amplitude of Pn wave on the simulated vertical Wood‐Anderson seismogram in nanometers, Δ is the epicentral distance in kilometers, 1.86 represents amplitude attenuation, C is the station correction, and 1.62 anchors the mb(Pn) to the moment magnitude (Mw), plus amplitude loss in the two crustal legs. By using moment magnitude as reference, we obtained the event magnitude adjustments (EMAs), which represent the difference between the known long‐period Mw and the short‐period mb(Pn). EMAs correlate with the source property, where ridge events show strong long‐period waves but poor high‐frequency signals that lead to mb(Pn) < Mw and positive EMA of +0.08 ± 0.26 magnitude units (m.u.), whereas transform fault earthquakes show relatively strong high‐frequency waves that lead to mb(Pn) > Mw and negative adjustment of −0.20 ± 0.24 m.u. Almost all normal faulting events are along spreading ridges, whereas strike‐slip events are along the fracture zones and transform faults; hence, we developed source‐specific magnitude adjustments (SSMAs), which range from +0.27 m.u. for earthquakes along the ridges, to −0.26 m.u. for earthquakes along the transform faults. Using SSMA, we obtained a regional Pn magnitude that is consistent with Mw and mb(P). The regression relationship is Mw = 0.91 mb(Pn)[SSMA] + 0.46 with a standard deviation of ±0.15 m.u.