The pair apparently arrived on the Hawaiian island of Kauai over the weekend to attend friend Ben Stiller's 40th birthday party, People reports.
Theroux and Aniston spent Tuesday visiting several of Kauai's popular attractions, with one eyewitness revealing: "They are acting like regular tourists... Justin is beyond sweet and attentive to her."
Last month, Aniston was apparently introduced to Justin's cousin Louis during the couple's trip to London.
The stars have yet to publicly confirm their romance, but Aniston recently admitted during a Good Morning America interview that she's "very happy" with her private life.
Aniston and Theroux star in the upcoming film Wanderlust, which opens on October 7 in the US and on November 25 in the UK.
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Watch Jennifer Aniston in the Horrible Bosses trailer below: