Waldo Porter Johnson (September 16, 1817 - August 14, 1885) was a United States Senator from Missouri. Born in Bridgeport, Virginia, he attended public and private schools, graduated from Rector College (Pruntytown, Virginia in 1839, studied law and was admitted to the bar, commencing practice in Harrison County, Virginia in 1841. He moved to Osceola, Missouri in 1842 and continued the practice of law, and served in the Mexican War as a member of the First Missouri Regiment of Mounted Volunteers. In 1847 he was a member of the Missouri House of Representatives and was elected circuit attorney in 1848 and judge of the seventh judicial circuit in 1851; he resigned in 1852 and resumed the practice of law.
Johnson was a member of the peace convention of 1861 held in Washington, D.C., in an effort to devise means to prevent the impending war; he was elected as a Democrat to the U.S. Senate and served from March 17, 1861, to January 10, 1862, when he was expelled from the Senate for disloyalty to the government. He served in the Confederate Army during the Civil War and attained the rank of lieutenant colonel of the Fourth Missouri Infantry, and was appointed a member of the Senate of the Confederate States to fill a vacancy.
From August 1865 to April 1866, Johnson resided in Hamilton, Canada; he returned to Osceola and resumed the practice of his profession; he was president of the State constitutional convention in 1875 and in 1885 died in Osceola. Interment was in Forest Hill Cemetery, Kansas City, Missouri.
Waldo Johnson was a nephew of Joseph Johnson, a U.S. Representative from Virginia.