John W. Donnellan (June 8, 1841 – July 26, 1917) was an American politician who served as the 1st Treasurer of the Wyoming Territory as a Democrat.

John W. Donnellan was born on June 8, 1841, in County Clare, United Kingdom of Great Britain and Ireland and his family immigrated to the United States and settled in Cincinnati, Ohio.

On December 21, 1869, President Ulysses S. Grant appointed him as treasurer of the Wyoming Territory and served until October 26, 1872.

In 1870 he married Marion J. Donnellan and moved back to Laramie in 1876 where he served as a probate judge.

Afterwards he moved to Salt Lake City and worked for a banking company and then to Sacramento in 1903 due to his wife's poor health.