Arthur John Rosier (October 31, 1880 – April 15, 1932) was an American lawyer and politician who served in the Wyoming Senate from 1927 until his murder in 1932.

He was the third of five children of wealthy farmer and inventor Thomas R. Rosier, and Helen M. Wolcott, both of whom were New York residents who had moved to Minnesota.

Arthur Rosier was raised in Sioux Falls, South Dakota, where he attended the local public school.

On June 6, 1906, Rosier was admitted to the State Bar of Montana and he opened a law practice in the city of Butte the following month.

[12][13] Bourquin was nominated for the position two years later by President William Howard Taft and subsequently confirmed by the United States Senate.

[1]In 1917 and 1918, Rosier was the prosecutor during the highly publicized murder trial of John Leibig, a naturalized German-American rancher who was accused of shooting his neighbor during a land dispute.

Despite this, after making a series of statements in support of the German Empire, Leibig was tried and convicted in federal court for violating the Espionage and Sedition Acts.

As a result of a strong campaign against him, Rosier was defeated in his 1920 re-election bid by Democratic nominee Revelle McMicken.

[28] In the 1927 legislative session, Rosier served on the Judiciary, Education, County Affairs, Mines, Engrossment, and Public Utilities committees.

[30] Additionally, Rosier also led arguments against a bill which would define and outlaw fraudulent conveyances, arguing the proposed change would lead to women being charged of a crime when husbands transferred land to their wives prior to becoming insolvent.

[36] During this session, Rosier was also part of an investigatory committee that investigated the state department of law enforcement over alleged crimes, financial mismanagement, and a "lack of diligence and honesty".

[41] Along with Democratic senator Bayard Wilson, he co-sponsored a bipartisan bill that would amend the state highway budget.

[42][43] In February 1931, at the end of the short legislative session, Governor of Wyoming Frank Emerson died and secretary of state Alonzo M. Clark succeeded him.

[45] On April 15, 1932, while Rosier was inside the Rawlins National Bank, he was shot and killed by Thomas Lacey, a local gambler.

[46][47] Lacey had followed Rosier into the bank just after 1 p.m. and shot him once in the back with an automatic revolver,[48] piercing his heart and instantly killing him.