The published final results confirmed President Macky Sall's BBY won 82 out of 165 seats, one short of the 83 required for an absolute majority, while the alliance of opposition coalitions Yewwi Askane Wi (YAW) and the Wallu Senegal won 80.
Lawmaker Pape Diop of the Bokk Gis Gis Liggeey coalition, who won one of the three remaining seats, said he would join forces with BBY to prevent a potential legislative crisis.
"We must prevent this situation ... from blocking the operations of our institutions," Diop said in a statement.
There was no immediate comment from the opposition on his decision.
"We should be proud and respect this choice of the voters who independently decided to rebalance the current political forces," Diop said.
BBY's slender majority contrasts with the 125 seats it won in the last parliamentary poll in 2017. Its loss of 43 seats reflects growing public acrimony towards the president, fueled in part by his refusal to state clearly whether he plans to run for a third mandate in 2024 in breach of term limits.