James S. Rains (October 2, 1817 – May 19, 1880) was a senior officer of the Missouri State Guard (MSG) who fought in the Trans-Mississippi Theater of the American Civil War.

The following year, President of the United States James Knox Polk appointed Rains as an Indian agent.

Early in the following year, he was removed from his position as Indian agent because of alleged misconduct at the Neosho Agency post.

[7] After the Missouri State Guard was defeated by Union Army forces at the Battle of Boonville, MSG commander Sterling Price ordered Rains and Brigadier General William Y.

[10] On July 5, the Battle of Carthage, Missouri, was fought, pitting the MSG against Union Army forces led by Franz Sigel.

In the words of historians David C. Hinze and Karen Farnham, Rains was "unable to conceptualize battle beyond a regimental scope" and "did little to help the confused situation and in fact made it substantially worse".

[20] Early in the morning of August 10, Rains was informed that a forage train had sighted Brigadier General Nathaniel Lyon's Union forces advancing against the camp.

[21] The first major response to Lyon's approach resulted from the individual initiative of one of Rains' subordinates, Colonel James Cawthorn.

[22] Rains' cavalry were among the first hit by Lyon's attack, and while some conducted a fighting retreat, others joined camp followers in what an eyewitness referred to as a "panic-stricken drove".

[24] Of the MSG units present at Wilson's Creek, Rains' division suffered the lowest casualty rate.

Rains made it to Stockton, where he incorrectly determined that the Kansans were more than his force could handle, so Price brought up the rest of the MSG and defeated them in the Battle of Dry Wood Creek on September 2.

After brief fighting, Rains' men withdrew, but Price, after receiving ammunition supplies, surrounded the Union garrison on September 18.

[27] The garrison surrendered on September 19 after the MSG advanced up the hill, pushing waterlogged hemp bales in front of them as protection.

[28] In early March 1862, Rains' MSG division fought in the Battle of Pea Ridge, attacking Union positions near Elkhorn Tavern.

[32] According to historian William L. Shea, Rains spent his time in command at Elkhorn Tavern "playing cards and consuming copious amounts of whiskey".

[33] Union attacks in September and October drove the outlying Confederate cavalry back, with Rains and the infantry making a difficult withdrawal across the Boston Mountains as well.

[34] Historian Mark K. Christ stated that Rains and Douglas H. Cooper "bungled the situation in northwestern Arkansas".