Catharina Oostfries, or Trijntje Sieuwerts (1636–1708) was a Dutch Golden Age glass painter.
According to the RKD she was the daughter of Siewert Oostvries, and was also known as Trijntje Siewerts. She was trained by her older brother Jozef Oostfries, a respected glass painter in Hoorn. She married the glass painter Claes Pietersz van der Meulen, and their son Sieuwert van der Meulen also became a painter. Her sister Heiltje married the painter Jan Slob, who had been a pupil of her brother. She died in Alkmaar.
- (Dutch) Catharina Oostfries Biography in De groote schouburgh der Nederlantsche konstschilders en schilderessen (1718) by Arnold Houbraken, courtesy of the Digital library for Dutch literature
- Catharina Oostfries in the RKD
- Heiltje Sieuwerts on genealogie online
- Jan Slob Biography in De groote schouburgh