The striker picked up a knock during his side's Bundesliga win on Saturday, but has now assured fans he will be ready for Wednesday's European encounter.
Die Borussen take on los Blancos in the first leg of their Champions League semifinal on Wednesday and Lewandowski, who has scored in each of his last 12 competitive matches, is certain he will be ready for the crunch European encounter.
"I wanted to reassure you that my appearance on Wednesday is not endangered," the Poland forward posted on his Facebook page.
"The muscle is bruised but I'll be fit for the match against Real!"
Lewandowski had earlier complained about pain in his thigh after his team's 2-0 victory against Mainz and originally thought he would struggle to make BVB's European tie.
"I felt pain in the last minute. All I can do is apply ice to the thigh and hope it doesn't swell anymore. It's touch and go whether I play on Wednesday," he was quoted as saying on UEFA.com.
Dortmund meets Jose Mourinho's men on April 24 at Signal Iduna Park.