Trump praised Netanyahu as a "great warrior prime minster" and peace partner, crediting him with joining the United States in launching strikes on Iran in February.
The conflict, in its seventh month, has disrupted energy markets globally and wreaked havoc across the Middle East, damaging critical infrastructure and destabilising regional economies.
"Bibi Netanyahu has been Strong, Brave, and Brilliant in the face of tremendous opposition, including that coming from even Israel, itself," Trump wrote in a post on Truth Social.
"Despite all of that, he has endured and, in fact, he has thrived! I have watched Bibi under the most trying of times. Normal men or women in such situations would have wilted under the pressure - But Bibi did not wilt, he actually got stronger, and maintains that same Strength today."
In his message, addressed to "the Great People of Israel," Trump added, "Please let this Letter serve to represent my Complete and Total Endorsement of Bibi Netanyahu in his upcoming Race for Re-Election as Prime Minister of Israel."
Trump's endorsement is not certain to give Netanyahu and his conservative Likud party a boost in the October 27 general election, as Israeli public support for Trump has diminished amid Israeli-US disagreements over Iran.
Polls have suggested no candidate has a clear path to power.
Israel's prime minister is chosen through coalition-building in parliament rather than by direct popular vote.
It will be Israel's first vote since the Hamas attack on October 7, 2023, that shook the country and plunged the Middle East into turmoil, as Israel went to war in the Gaza Strip, Lebanon and Iran.
The election could determine whether Netanyahu remains in power after one of the most consequential periods in Israel's history, or is defeated by centrist rivals promising to usher in a new era.
Netanyahu promptly expressed his thanks to Trump on X, saying, "Together we will complete the mission!".
Trump endorsed Netanyahu in past elections and in November he sent a letter to Israeli President Isaac Herzog asking him to pardon Netanyahu on corruption charges.
But Trump's comments about Netanyahu in recent months had been much more subdued and he repeatedly demurred when asking if he would back the prime minister.
Trump has endorsed candidates in elections in other countries including Hungary, Colombia, Argentina, Japan and Honduras.
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