Coordinates: 47

° 36 23 .3 N, 122 ° 19 31 .6 W / 47. 606472, -122.325444 Archdiocese of Seattle (Latin Archidioecesis Seattlensis) - The Roman Catholic archdiocese, with its capital i n Seattle, Washington, United States. Is the most important metro polis of Seattle Diocese, which coincides with the territorial state of Washington. The diocese was established May 31, 1850 as the Diocese of Walla Walla. September 11, 1907 changed its name to the Diocese of Seattle. December 17, 1913 Spokane Diocese separated.