The 20-year-old goalkeeper opted for the three-year contract in his Czech homeland after failing to break through from the Blues' reserve sides.
"For Jan it was a great experience to be at Chelsea," his agent told a Czech sports website. "He has played regularly for the reserve team and a few times he has been involved with the first team and taken part in training sessions.
"The time has now come for him to attract more attention. He is bound to be an important member of the Czech U21 side and that is why we have decided to go with Jablonec, a side that traditionally gives young players an opportunity to shine."