This perhaps marks the highest profile snub of his party's standard-bearer, Donald Trump.
Powell, who also served in the Reagan and George H.W. Bush administration announced his plans at a Long Island group during a luncheon.
Then, in case anyone misunderstood Powell's announcement, it was was confirmed by his chief of staff, Peggy Cifrino.
Grateful for his support Clinton tweeted...
"Proud to have the endorsement of General Powell, a decorated soldier and distinguished statesman,"
The tweet was signed with an "H" indicating that she wrote the post herself.
The endorsement came as a semi-surprise since hacked emails released in September revealed that Powell has had tough words for both major party candidates.
Though Powell leveled some harsh critiques at fellow Secretary of State Clinton he unabashedly blasted her rival Donald Trump labeling him a "national disgrace and an international pariah" and even stating in one email that "Trump is nuts".
Despite Powell's support for Clinton, as well as endorsing President Obama twice, he insists that he is still a Republican...
"I'm still a Republican because I believe in a strong defense, because I believe in the entrepreneurial spirit that is so typical of the Republican Party in the past. But, I'm having difficulty with the party now."
Powell is going to have to get in a long line if he want's to join that club of other high-profile Republicans that have thrown their support behind Hillary Clinton. A list that includes: Former Sen. Larry Pressler, Sally Bradshaw (senior adviser on Jeb Bush's campaign) and Hewlett Packard CEO Meg Whitman and ex-Reagan political director Frank Lavin and Former President George H.W. Bush to name a few.