The midfielder connected sweetly with a right-footed shot from 20 yards and saw the ball fly into the top corner of the net.
City went ahead with a fluke goal after 12 minutes. Sam Baldock controlled Marlon Pack's pass on the right touchline and saw his attempted cross elude goalkeeper Owain Fon Williams, the ball going in off the far post for the striker's 17th goal of the season.
But Tranmere had started well and were level seven minutes later when James Wallace crossed from the left and Jake Cassidy turned past Aden Flint to drive a fierce shot over goalkeeper Simon Moore from 10 yards.
The hosts thought they had won it in the 78th minute when Baldock's chip from the right to the far post was headed home by left-back Derrick Williams for his first City goal.
But the least Tranmere deserved was a point on the balance of play and the classy Koumas had the final say.
- Sports & Recreation
- Sam Baldock
- Owain Fon Williams
- Jake Cassidy