Hanna Wilenius soaked the leader earlier this year as a stunt for a television comedy. Now she has been fined 100 days' wages for harassment, The Local reported.
The court said Wilenius should have taken into account the fact Sweden has previously suffered two ministerial assassinations. The court also said she should have realized there was the possibility the joke could be taken the wrong way.
Fredrik Reinfeldt testified in court via telephone.
"I wasn't prepared and was taken aback by what happened," he said. "I didn't find it funny when the water was squirted."
Wilenius said she "never intended to offend" Reinfeldt. Her lawyer, Percy Bratt, said she would appeal the verdict.