[1] Mid-ocean ridge volcanic activity is the fundamental process for creation of ocean crust, yet the dynamics of magma emplacement along the slow spreading Mid-Atlantic Ridge ( MAR) are largely unknown. We present acoustical, seismological, and biological evidence of a magmatic dike intrusion at the Lucky Strike segment, the first detected from the deeper sections (> 1500 m) of the MAR. The dike caused the largest teleseismic earthquake swarm recorded at Lucky Strike in > 20 years of seismic monitoring, and one of the largest ever recorded on the northern MAR. Hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband (> 3 Hz) intrusion tremor, suggesting a volcanic origin. Two submersible investigations of high-temperature vents located at the summit of Lucky Strike Seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting. This magmatic episode may represent one form of volcanism along the MAR, where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long-lived, discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems.