The dismissal will take effect once an appeal against it has been ruled on.
"I see no basis to deny," Judge Michael Obus said.
The decision in New York state court brought the curtain down on a three-month saga that destroyed the career one of the world's most powerful men and caused an international uproar.
Obus also issued a ruling denying a request by lawyers for the maid accusing Strauss-Kahn to have the investigation transferred to a special prosecutor.
The dismissal of all charges against Strauss-Kahn will only take effect once the appeal of the special prosecutor decision has been dealt with, Obus said.