The 70-year-old reality TV star is "ignorant" of the complex challenges that the US faces and has not shown any interest in "educating" himself, they said.
"(Trump) is entirely unqualified to serve as President and Commander-in-Chief.
"He is ignorant of the complex nature of the challenges facing our country, from Russia to China to ISIS to nuclear proliferation to refugees to drugs, but he has expressed no interest in being educated," the former ambassadors and diplomats said.
Former Assistant Secretary of State for South and Central Asia Robert Blake, who till recently was the US Ambassador to Indonesia, are among the signatories to a letter endorsing Clinton.
The letter said Trump "entirely misunderstands and disrespects" the high officials in the foreign service and intelligence communities that could guide him to a right course of action on crucial questions of foreign policy.
"Hillary Clinton's handling of foreign affairs has consistently sought to advance fundamental US interests with a deep grounding in the work of the many tens of thousands of career officers on whom our national security depends," the letter said.