The 31-year-old is currently playing in the Indian Premier League for Royal Challengers Bangalore, alongside Sussex and England pace bowler Chris Jordan.
Wiese has made 20 T20 appearances for South Africa, including two against England earlier this year.
He will be available to play for Sussex against Somerset on Wednesday, 1 June and Surrey two days later.
Sussex have signed South Africa all-rounder David Wiese for their first two home T20 Blast matches.