The 28-year-old is available to face Hampshire, Surrey, Somerset and Essex in the last month of the campaign
Northamptonshire have acquired Lizaad Williams for the climax to the LV= Insurance County Championship in September
Four games remain for John Sadler's side, who sit fifth in Division One.
Williams, who arrives in the UK this week, is available for the final flurry of matches, starting against Hampshire on Monday (September 5).
They then host Surrey (starts September 12), travel to Somerset (September 20) before ending the campaign at home to Essex (September 26).
Lizaad Williams - First-class career stats
Matches: 67
Wickets: 209
Average: 27.08
Best innings figures: 7 for 75
Best match figures: 11 for 106
Vredenburg-native Williams has played two Tests, a single one-day international and six T20Is for South Africa.
He possesses a credible first-class record comprising 67 matches, during which he has collected 209 wickets at 27.08.
"I'm looking forward to joining up with the squad and to hit the ground running," Williams said.
"The team is in a strong position in the County Championship and I hope I can contribute towards some positive results in the upcoming games.
Williams was part of The Proteas' ODI squad for the England series earlier this summer, but only played against the Lions (Nathan Stirk/Getty Images)
"I've been following the team since two former teammates of mine Rory Kleinveldt and Richard Levi plied their trade for Northamptonshire.
"I've played with Vasconcelos and Glover down in Cape Town too so I’m looking forward to seeing them again."
The news follows confirmation that Chris Lynn will return to Northants for next season's T20 Blast and allrounder Josh Cobb has penned a new contract until 2024.
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