He begins a two-year term in January.
Harper, of Pearl, first won the seat November 2008. The 56-year-old says he's focused his on his work on the Energy and Commerce Committee.
Crystal Biggs of Florence won the Democratic primary but withdrew from the race for health reasons. Democrats didn't replace her on the ballot.
Reform Party candidate John "Luke" Pannell ran in the 3rd District but never filed a campaign finance report.
The 3rd District stretches from Wilkinson County by the Mississippi River, through the Jackson suburbs in Rankin County and up to Noxubee County, bordering Alabama. It includes 20 counties and parts of four.