“That seems like revenge and it is,” Schmidt told weekly Die Zeit's magazine in an interview to be published Thursday.
Schmidt, who served as West German chancellor from 1974 to 1982, frequently weighs in on topical issues despite having reached the advanced age of 91.
The US government is examining whether it can prosecute Assange, an Australian national, over the release of thousands of classified US documents diplomatic cables on the internet and through major newspapers.
US Attorney General Eric Holder has said officials were pursuing a "very serious criminal investigation" into the matter.
Assange, the founder and front man of the whistleblower website Wikileaks, is currently free on bail in Britain for alleged sex offences in Sweden.
Schmidt said there was a “moral obligation for the publication” of secret documents that exposed abuses at prisons such as Abu Ghraib or Guantánamo Bay, but questioned whether secret diplomatic communications should be released.
“It must still be possible that a conversation two people have with one another remains confidential,” he said.
He said he had “understanding, but no sympathy” for the newspapers that had published the Wikileaks diplomatic cables.
For the US government, the publication of the cables was “a terrible thing, which will affect diplomatic relations for a long time.”
“But the Americans will get over it,” he said.