The abundance of the sei whale Balaenoptera borealis in the central and eastern North Pacific (north of 40A degrees N, south of the Alaskan coast including both the US and Canadian Exclusive Economic Zones between 170A degrees E and 135A degrees W), from July to August, was estimated by the line transect method using sighting data obtained during the 2010-2012 International Whaling Commission-Pacific Ocean Whale and Ecosystem Research cruises. The probability of detecting whales at a perpendicular distance from the transect was estimated using two different models: hazard rate and half-normal models. Because the difference in Akaike's information criterion between the two models was small, the Akaike weighted average of the two models was taken, which gave an estimated abundance of 29,632 (coefficient of variation, 0.242; 95% confidence interval, 18,576-47,267). This is the first abundance estimate of sei whales in this region based on systematic sighting survey data, which contributes to an understanding of the current status of this species.