Kalidou Koulibaly's excellent volley 20 minutes from time clinched a vital three points in this winner-takes-all encounter in Group A.
It means that Senegal leapfrog Ecuador into second place, finishing on six points behind the Netherlands.
The Lions of Teranga will now face the winners of Group B, which will be settled later on this evening, and it could still be any one of the four sides in the section.
After a dominant display in the first half, Senegal were given a minor shock when they were pegged back midway through the second half but wasted no time in re-taking the lead, and then held on for a fantastic victory.
It is just the second time that Senegal have made it through the group stage at the World Cup, with the last occasion coming in 2002, when they would go on to reach the quarter-final.
It has eliminated the horrors of 2018 when they were knocked out at this stage on the fair play rule after finishing with an identical record to Japan.
Aliou Cisse made a few changes to his Senegalese side, as a more cautious approach was implemented following their game against Qatar, with attackers Famara Diedhiou and Krepin Diatta both making way for Iliman Ndiaye and holding midfielder Pathe Ciss.
That did not affect the potency of their offensive display in the first half though, as they still created numerous chances, with Ecuador hardly threatening Edouard Mendy's goal at all in the first half.
Idrissa Gueye and Boulaye Dia were both guilty of missing gilt-edged chances inside the opening eight minutes, but the pressure continued to rise on the Ecuador goal as the half went on.
Senegal eventually took the lead shortly before half time, when Piero Hincapie barged Ismaila Sarr down in the penalty area, with French referee Clement Turpin subsequently pointing to the spot.
Sarr picked himself up to take the penalty, and he calmly slotted it beyond Hernan Galindez into the bottom right-hand corner.
In the second half, Senegal seemed content with holding what they had, and while Ecuador were still showing little threat, Cisse's side were undone by a set piece.
After Felix Torres's flick on from a corner, Moises Caicedo was left all on his own at the back post with the most simple of tasks to slide the ball beyond Mendy for the equaliser.
Ecuador would be in the ascendancy for just two minutes though, as the African side retook the lead from a set piece of their own, after Enner Valencia failed to clear from Gueye's dangerous delivery, giving Koulibaly the chance to volley home from close range.
Despite throwing everything at the Senegalese goal in the final quarter of the game, Ecuador were unable to break through once again, and fell to defeat, suffering elimination even though they only needed a point before the game to progress.
Senegal will now have a knockout fixture to look forward to, and with England currently the favourites to be their next opponents, progressing even further in this tournament will cap off what has already been a historic year for the national side, which started with them being crowned Africa Cup of Nations winners.