Andrew Jackson Thayer (November 27, 1818 – April 28, 1873) was an attorney and Democratic U.S. congressman from Oregon.

After Congress, Thayer returned to legal work and served as a district attorney and justice of the Oregon Supreme Court.

[1] In 1859, he was appointed by President James Buchanan to be U.S. Attorney for the District of Oregon, a position he held for six months, after which he resigned, stating that he preferred to defend rather than prosecute.

[1] In November 1860, Thayer was elected as a Democratic United States Representative for Oregon's at-large district.

[4] Representative Thaddeus Stevens of Pennsylvania agreed with Thayer's argument that this decision was in violation of Article I, Section 4 of the United States Constitution, which gives the power to the state legislatures to set election dates.

[5] The House stripped Thayer of his seat and issued the oath of office to Shiel immediately.