Hilton, a socialite and occasional actress, and Reinhardt, a baseball player and reality television personality, reportedly dated for several months before they went their separate ways in June.
"They are definitely happy again," an insider told Usmagazine.com Friday.
Although his publicist declined to comment on the story, Reinhardt posted on Twitter Monday: "What an amazing weekend with my beautiful girlfriend. I love her so much!"
"Laying in bed watching SportsCenter with my girl," he tweeted the following day.