Whitman County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 44,776. The county seat is Colfax, and its largest city is Pullman. The county was formed out of Stevens County on November 29, 1871. It is named after Marcus Whitman, a Presbyterian missionary who, with his wife Narcissa, was killed in 1847 by members of the Cayuse tribe.