Not since winning the title in 2012 had Toulouse reached a final, but it is just one victory away from lifting the trophy again.
Sofiane Guitoune put his side in front with a try in the 13th minute, with Thomas Ramos adding the extras.
Two penalties within five minutes from Ihaia West brought La Rochelle to within a point of Toulouse at the interval, but the game was taken away from the European Challenge Cup runner-up in the second half.
Sebastien Bezy crossed on the hour-mark and another try from Cheslin Kolbe followed up within five minutes.
Bezy added further gloss with a penalty in the 73rd minute, as Toulouse secured its first Top 14 final appearance in seven years, with Clermont Auvergne its next opponent in the final after it beat Lyon in the second semi-final.