Injuries to reserve goalkeepers Fabian Giefer (abdomen) and Louis Hulsmann (knee) mean Jens Keller's side are lacking experience to support first-choice Ralf Fahrmann, with only 18-year-old Timon Wellenreuther available to the head coach.
Wetklo's arrival will ease Keller's problems and aid the development of the younger goalkeepers at the club.
The 34-year-old came through the youth system at Schalke before building a career with Mainz, where he made 114 Bundesliga appearances.
Wetklo was a free agent at the end of last season and had agreed to join Darmstadt in the second tier, but he terminated that contract before taking up a "dream" deal with his boyhood club.
"Once Schalke, always Schalke," said Wetklo. "It fulfils a lifelong dream for me one more time, to return to my sporting home after 15 years.
"It's an overwhelming sense of happiness to be able to play for the club again, whose games I followed since I was six.
"With the tasks and role that Schalke 04 proposed to me, I was always 100 per cent sure to come back to Schalke."
General manager Horst Heldt has confirmed that Wetklo will move to the club's under-23 side when Giefer and Hulsmann return to full fitness, while always being available as back-up for the first team.