After 80 minutes of play, the encounter looked to be ending for a draw before the 20-year-old sent his team’s fans into a frenzy with his 81st-minute effort.
Benjamin Andre released Sarr who found space in the goal area of the visitor before firing past a helpless goalkeeper Ludovic Butelle.
Angers rallied in search of an equaliser, but Rennes held firm to earn their first triumph of the season.
The Senegalese was on parade for the entire duration of the game as well as Mali international Hamari Traore.
His compatriot, Diafra Sakho was handed a starter’s shirt but was replaced in the 87th minute by Damien Da Silva.
With the victory, Rennes are now ranked 12th in the table and they face Olympique Marseille in their next encounter.