Non-volcanic tremor accompanies slow slip events at most circum-Pacific subduction zones. Previously, the Hikurangi subduction margin, New Zealand, and the Boso Peninsula of Japan have been noted as exceptions where slow slip, which occurs at shallow depth compared to other subduction zones, does not generate seismic tremor. The lack of tremor observations raises questions whether tremor and slow slip are manifestations of the same process and whether different temperature and pressure conditions influence the physical mechanism of tremor production. Analysis of seismic data during the March 2010 Gisborne slow slip event, New Zealand, reveals that this slow slip event at the northern Hikurangi margin is accompanied by tremor with distinctive characteristics compared to local microearthquakes. Both the Gisborne area of New Zealand and the Boso Peninsula of Japan are underlain by thick accumulations of sedimentary rocks that likely inhibit the observation of low amplitude tremor due to high attenuation. Discovery of tremor in New Zealand suggests that tremor may always accompany slow slip and that its apparent absence at the Boso Peninsula, the Hikurangi margin prior to this study, and any future regions discovered to experience slow slip without concurrent tremor, may be the effect of highly-attenuating sediments.