Immobile enjoyed a stellar Serie A campaign, winning the league's golden boot with 22 goals as his side claimed a European place due to Parma's ineligibility.
Speaking after playing 57 minutes for Italy in their 0-0 draw against the Republic of Ireland on Saturday, the 24-year-old said his move to Dortmund was imminent.
"It’s done (the Dortmund deal), I said it a short while ago," Immobile told RAI Sport.
"All that's waiting to be done is for me to sign the contract."
Immobile expressed his sympathy for midfielder Riccardo Montolivo, who is out of the FIFA World Cup after suffering a broken leg in the Ireland draw.
Montolivo was substituted in just the 15th minute, and Immobile felt the injury had a major impact on Italy at Craven Cottage.
"I'm very sorry for Montolivo, we knew how much he was looking forward to the tournament," Immobile said.
"We hope he can recover soon.
"The injury disrupted us a little, but it can’t be an excuse. We had some heavy legs and the emotion from what happened was weighing heavily on us."
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