The former Brighton defender, 33, who managed Worthing from 2015 for just over two seasons, had been searching for a new club after leaving Burgess Hill last month.
But the centre-half has found a new home and will be intent on helping U's secure a play-off spot.
Following the arrival of experienced ex-Albion defender Elphick, Hastings manager Chris Agutter told the club website: “We're very pleased to finally welcome Gary on board. It’s no secret that we’ve needed an experienced centre-back for some time.
"To have acquired the services of a player of Gary’s stature and experience, and the impact that will have on our young dressing room can’t be underestimated.
Bringing in a player of Gary’s calibre only reinforces and helps facilitate the club’s belief in developing local talent, as his experience and knowledge can only benefit those around him, both on and off the pitch.”
The 33-year-old made two Brighton appearances at the start of his career.
Elphick enjoyed a spell as Worthing manager, guiding the club to promotion from the then-Ryman League South through the play-offs when joint boss alongside Jon Meeney in 2016.
After resigning as manager of the Woodside Road outfit a year later, Burgess Hill came calling and Elphick made 26 appearances for them before leaving last month with manager Simon Wormull saying he required ‘some time away from the game’.
Have you read?