The 23-year-old, who represented England at Under-20 level, has signed a deal with the capital outfit until 2015.
"Seb is a good recruit for us; he'll add a lot of strength to our squad. I'm really looking forward to working with him," Jones told the club's official website.
"He is another successful product of the Harlequins Academy. He's had a very promising career up till now and will benefit from playing more games."
Stegmann made his first-team debut for Harlequins in 2007.