Trachtenberg, who is well known for her role as Georgina in Gossip Girl, will appear in two episodes later this season.
She will play Emma, who is described as attractive and clever.
Emma is a drug dealing rival to Nancy (Mary-Louise Parker) and is noticed by Silas (Hunter Parrish).
As well as starring in Gossip Girl, Trachtenberg played Dawn in Buffy The Vampire Slayer and has appeared in shows including Six Feet Under, Mercy and Love Bites.
Martin Short, Aidan Quinn and Lindsay Sloane will also feature in the new season of Weeds, which premiered last night.
The show's star Justin Kirk recently admitted that he does not expect the series to end this year, although executive producer Roberto Benabib has suggested that the new season may be the last.
Weeds continues on Mondays at 10pm on Showtime.