Warner, 22, is returning to Hove after his spell there earlier this summer was ended by a side strain.
The Wakefield-born Warner will link-up again with former Yorkshire coach Jason Gillespie and is available for Sussex’s last four County Championship games, starting with Sunday’s match at home to Middlesex.
Sussex are third-bottom of Division Two but are only 29 points behind second-placed Glamorgan as Gillespie’s men seek a strong end to the campaign.
Warner, who has lately been playing T20 cricket for Yorkshire’s second team, said: “I’m looking forward to it (going back to Sussex).
“I enjoyed my time there earlier in the year and I'm just looking forward to hopefully playing some more first-team cricket.
“If I can get a couple more games under my belt, I think it will really improve my game going forward into next year.
“Obviously I’m desperate to play for Yorkshire going forward, so anything I can do that will help my cause is what I want to do.”