William Paul Dillingham (December 12, 1843 – July 12, 1923) was an American attorney and politician from the state of Vermont.

Groomed for a political career from an early age, Dillingham served as Secretary of Civil and Military Affairs (chief assistant to the governor) during his father's term and that of Asahel Peck, State's Attorney of Washington County, and member of the Vermont House of Representatives and Vermont State Senate.

[2] He attended the public schools, Newbury Seminary in Montpelier, Vermont, and Kimball Union Academy in Meriden, New Hampshire.

[7] Dillingham was elected to the United States Senate in 1900, replacing temporary appointee Jonathan Ross and completing the term of Justin Smith Morrill, who had died in office.

Dillingham achieved prominence as the leading legislative spokesperson for restricting immigration during the Progressive Era.

From 1907 to 1911, Dillingham chaired (concurrently with his Senate duties) the United States Immigration Commission, also called the Dillingham Commission, which concluded that immigration from Eastern and Southern Europe posed a serious threat to American society and culture and should be greatly reduced in the future.

While the subcommittee traveled extensively throughout Alaska, Dillingham never set foot in the Bristol Bay salmon fishing community that still bears his name.

[10] Dillingham's business interests included serving as President of the Waterbury Savings Bank and a member of the National Life Insurance Company board of directors.

[13][14] Dillingham was a member of the Sons of the American Revolution, and served as President of the Vermont SAR organization.