William Dennison, Jr. (November 23, 1815 – June 15, 1882) was a Whig and Republican politician from Ohio. He served as the 24th Governor of Ohio.

Born in Cincinnati, Dennison graduated from Miami University, studied law, and was admitted to the Ohio bar in 1840. A canny businessman, he led the Exchange Bank and the Columbus and Xenia Railroad, and organized the Hocking Valley Railroad, while becoming active in politics.

Dennison was one of the first major Ohio politicians to leave the dying Whig Party for the new Republican Party. He rose quickly through the party ranks due to his anti-slavery and anti-discrimination efforts in the Ohio State Senate. Dennison was elected to the governorship in 1859, defeating Rufus P. Ranney , and served a single term from 1860 to 1862. Before the outbreak of the American Civil War, he refused the demands of Kentucky and Virginia state authorities for the extradition of fugitive slaves or the punishment of those who helped them. He organized Ohio's mobilization in the opening days of the war, and was generally effective, despite having a small staff and no prior military experience. Without being asked by the War Department, he sent Ohio troops into western Virginia, where they guarded the Wheeling convention which eventually led to the admission of West Virginia as a free state.

However, errors by the Governor and his subordinates led the state's alliance of Republicans and War Democrats to drop Dennison as a candidate in 1862. The parties turned instead to David Tod, a War Democrat. Historian Richard H. Abbott wrote, "No Ohio chief executive [before Dennison] had ever exercised such powers and fulfilled such duties with a greater sense of public responsibility and determination. Nevertheless...politics dictated his demise."

Dennison accepted this turn of events with good grace, capably advised his successor, and provided valuable services in helping recruit black troops for Ohio units. He was appointed U.S. Postmaster General by President Abraham Lincoln, and served from 1864-1866, leaving the Cabinet after he decided he could no longer support the policies of President Andrew Johnson. Dennison remained active in state and national politics until his death. He left behind a widow and seven children.

Of Dennison's single term in the opening stages of the Civil War, historian John S. Stilt wrote, "His wisdom and foresight were appreciated by few and condemned by many.... It is doubtful whether any of his predecessors could have met the isues any more successfully."

Dennison tried but failed to be elected to the United States Senate in 1861, when he was defeated by John Sherman, and in 1880, when he was defeated by James Garfield.

|- style="text-align: center;" | width="30%" |Preceded by:
Montgomery Blair | width="40%" style="text-align: center;" |United States Postmaster General
1864–1866 | width="30%" |Succeeded by:
Alexander W. Randall