Robert Marcellus Stewart (March 12, 1815 – September 21, 1871) was the 14th Governor of Missouri from 1857 to 1861, during the critical years just prior to the American Civil War.

Stewart was born in Truxton, New York, but moved to Kentucky with his parents when he was a boy.

When Stewart left office in January 1861, he urged Missouri to adopt an armed neutrality in the impending Civil War and not to provide men or arms to either side though his own preference was for preserving the Union.

In his final message as governor, he said: Stewart's successor as governor, Claiborne Jackson, claimed to support the armed neutrality stance though he also stated that be believed Missouri's course was tied to the other slave states of the Confederacy.

Stewart remained a bachelor all his life and was considered quite eccentric, including a famous instance of riding his horse into the governor's mansion.