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Hundreds of earthquakes were recorded during a nine‐month ocean bottom seismometer deployment surrounding Lō'ihi submarine volcano, Hawai'i. The 12‐station ocean bottom seismometer network widened the aperture of earthquake detection around the Big Island, allowing better constraints on the location of seismicity offshore Hawai'i. Although this deployment occurred during a time of volcanic quiescence for Lō'ihi, it establishes an important basis for background seismicity of the volcano. Offshore seismicity during this study was dominated by events located in the mantle fault zone at depths of 25–40 km. These events reflect rupture on preexisting faults in the lower lithosphere caused by stresses induced by volcano loading and flexure of the Pacific Plate (Pritchard et al., 2007, https://doi.org/10.1111/j.1365‐246X.2006.03169.x; Wolfe et al., 2004, https://doi.org/10.1029/2003GC000618). Tomography was performed using double‐difference seismic tomography and showed shallow velocities to be slower than the regional velocity model (HG50; Klein, 1981, https://pubs.geoscienceworld.org/ssa/bssa/article/71/5/1503/118231/A‐linear‐gradient‐crustal‐model‐for‐south‐Hawaii). A broad, low‐velocity anomaly was observed from 20–40‐km depth, and is suggestive of the central plume conduit that supplies magma to Lō'ihi and the active volcanoes of the Big Island. A localized high‐velocity body is observed 4–6‐km depth beneath Lō'ihi's summit, extending 10 km to the north and south. Following Lō'ihi's active rift zones and crossing the summit, this high‐velocity body is characteristic of intrusive material. Two low‐velocity anomalies are observed below the oceanic crust, interpreted as melt accumulation beneath Lō'ihi and magmatic underplating beneath Hawai'i Island. Plain Language Summary: We deployed a network of ocean bottom seismometers on Lō'ihi submarine volcano for a nine‐month period in 2010–2011. During that time, there were few earthquakes recorded within Lō'ihi, but many in the surrounding area. These earthquakes are thought to be associated with the bending of the Pacific Plate under the weight of Hawai'i Island rather than volcanic processes. Data from these earthquakes were used to examine the speed of seismic waves within Lō'ihi, providing clues into the volcano's internal structure. We find that Lō'ihi has many of the same internal features as the older volcanoes on Hawai'i Island. Slower velocities surrounding Lō'ihi reveal the boundary between Lō'ihi and the underlying flank of Mauna Loa volcano. Slow seismic velocities within the mantle beneath the region may be associated with the Hawaiian hot spot itself. Key Points: An ocean bottom seismic network was deployed on Lō'ihi submarine volcano in 2010–2011Lō'ihi was seismically quiescent during the nine‐month deployment periodP wave tomography reveals low velocities beneath Lō'ihi and high velocities within its rifts and beneath the summit [ABSTRACT FROM AUTHOR]