We have discovered a prominent >190 km long, ~0.5--6 km wide lava channel on Io. The channel is sinuous with interior islands and may be associated with the active Tawhaki Patera hotspot. Photoclinometric analysis of the Galileo images indicates that this channel, provisionally named Tawhaki Vallis, is ~40--65 m deep. Although a constructional contribution cannot be ruled out, the depth, morphology, and sinuosity of the channel is consistent with erosion by lava. Erosion of flowing silicate over silicate substrate or flowing sulfur over sulfur substrate likely requires eruption durations of days to months to form an ~50 m deep channel, whereas flowing silicate over a sulfur substrate or flowing sulfur over a frozen SO2 substrate would likely require only hours to days. Future spacecraft observation of actively forming lava channels on Io are possible and desirable.