Spencer Jarnagin (1792 – June 25, 1851) was a United States Senator from Tennessee from 1843 to 1847.

Jarnagin was an elector for the Whig ticket of William Henry Harrison and John Tyler in the 1840 United States presidential election.

Known as the "Immortal Thirteen" by Tennessee Democrats, they refused to allow a quorum on the issue.

Jarnagin finally assumed office on October 17, 1843 and served until March 3, 1847.

The Whigs nominated him for a second term in 1847, but he was not elected, apparently the Democrats being more amenable to John Bell, another Whig who was eventually elected his successor; a subsequent campaign by Jarnagin for the Tennessee Supreme Court was likewise unsuccessful.