Semi-finalists last time out, Senegal secured all three points with first half goals from Ibrahima Niane and Cavin Diagne.
With the United States and Ecuador drawing 3-3 in Group F's other game, Senegal are the early pacesetters in Incheon.
The Senegalese could have scored more goals but found the Saudi Arabian goalkeeper Mohammed Alyami in inspired form.
Senegal next face the US on Thursday while Ecuador takes on Saudi Arabia.
Sixteen of the 24 teams advance to the second round - the top two from each of the six groups and the four best third-place finishers.
The final is scheduled for 11  June in Suwon
Click here for U-20 World Cup results from the Fifa website
Senegal began their Fifa Under-20 World Cup campaign in triumphant fashion by beating Saudi Arabia 2-0 on Monday.