The 48-year-old Belgian has previously worked alongside Riga at Standard Liege and French club Metz.
Jeunechamps was on the touchline as the Addicks beat Rotherham 4-1 in the Championship on Saturday.
Wim De Corte and Jason Euell both remain as first-team coaches - posts they held under interim coach Karel Fraeye, who was sacked last month.
Charlton Athletic head coach Jose Riga has appointed Jose Jeunechamps as his assistant at The Valley.