The Gossip Girl star, 31, was accused of sexual assault by three different women in November 2017.
Kristina Cohen and Aurelie Wynn accused Westwick of rape and Rachel Eck accused him of sexual assault – with all of the alleged incidents reported to have occurred in 2014.
Despite the first two accusers having identified witnesses, prosecutors said the witnesses were “not able to provide information that would enable the prosecution to prove either incident beyond a reasonable doubt”.
Investigators said the prosecution could not go ahead due to the “insufficient evidence”.
Westwick denied the rape allegations on social media last year.
“It is disheartening and sad to me that as a result of two unverified and probably untrue social media claims, there are some in this environment who could ever conclude that I have had anything to do with such vile and horrific conduct,” he said.
The actor added he was “cooperating with the authorities” in order to clear his name.
Ms Eck subsequently came forward and filed a complaint against Westwick – who rose to fame for his role as Chuck Bass on teen television drama series Gossip Girl.
The allegations disrupted Westwick’s working life and forced a re-shooting of scenes involving him in the BBC adaptation of Agatha Christie’s Ordeal By Innocence.
It also resulted in series two of his BBC Two show White Gold, where he plays Vincent Swan, being “paused”.
Westwick’s first accuser, actress Ms Cohen, claimed she woke up abruptly while sleeping in the actor’s guest room, with him on top of her.
He denied it, saying he did not know her.
Former actress Ms Wynn then came forward and said Westwick pinned down after she refused to have sex with him in an apartment in Los Angeles in 2014.
Ms Eck, a creative producer, said Westwick tried to kiss her “or kind of push me up a wall” and “aggressively groped” her in a hotel room.