Monaco slump again ahead of Champions League date
Monaco, runners-up in Ligue 1 to Paris Saint Germain last season, remain third from bottom of the table with just one victory -- achieved on the opening day of the campaign -- to their name.
Friday's loss was Monaco's second defeat of the week after they went down 1-0 to Angers on Tuesday.
That embarrassment prompted under-fire coach Leonardo Jardim to make seven changes to his starting line-up but the radical surgery had little effect.
Radamel Falcao wasted a handful of chances before Saint Etienne took a grip on the game.
Tunisian midfielder Wahbi Khazri opened the scoring four minutes before half-time when he latched on to a cross from Norwegian teammate Ole Selnaes to beat Monaco goalkeeper Diego Benaglio.
Khazri added a second in the 54th minute for his 40th career goal in Ligue 1 and his fourth of the season.
Saint Etienne are now second in the table, six points behind PSG who have a perfect seven wins in seven games.