Robert Morgenthau’s just not the retiring kind.

Barely three weeks after he left his nearly four-decade job as Manhattan district attorney, Morgenthau announced yesterday that he has joined the New York-based law firm of Wachtell, Lipton, Rosen & Katz.

Morgenthau retired as district attorney this month at age 90, after 35 years on the job.

Cyrus Vance Jr., elected to replace Morgenthau, took office on Jan. 1.