Altidore scored his first goals of 2014 for either club or country when his brace fired the United States to a 2-1 friendly win over Nigeria in Jacksonville.
The 24-year-old forward endured a tough season in the Premier League, making 31 appearances in the top flight for just one goal.
But Klinsmann claimed Altidore's confidence will sky rocket, after burying an international drought that had extended to six matches.
"Obviously it's a wonderful message to see that Jozy put the thing in the net," the German tactician said.
"It gives him a big smile at the right time now. It will give him a lot of confidence. It's always a tough period when you don't score for striker."
Klinsmann said he told Altidore he had to work harder before the rewards would come.
"The moment comes and you put it in there and then you start to be your own self again, but you've got to go through that working process," he said.
"That's what I always told him, you're not getting it for free. You've got to go and work for it.
"He got sharper and sharper, fitter and fitter and we knew that if he gets chances, he will put one in."
On their battle with fellow World Cup-bound opponents Nigeria, Klinsmann said his men took time to find rhythm.
"It took us a while to get into the game because we couldn't keep the ball, the first 20 or 25 minutes, long enough to play out the situation and switch sides," he said.
"Once we understood to make the field wider, to stretch it, we looked better.
"We started to have a better flow, better combinations. We started to play more simple."
The USA kick off their World Cup campaign against Ghana on June 16, before battles with Portugal and Germany.
