Catharina Oostfries, or Trijntje Sieuwerts (1636 - 1708) was a Dutch Golden Age glass painter.
Oostfries was born in Nieuwkoop. According to Houbraken she kept up her drawing and glass painting into her seventies.[1]
According to the RKD she was the daughter of Siewert Oostvries, and was also known as Trijntje Siewerts.[2] She was trained by her older brother Jozef Oostfries, a respected glass painter in Hoorn.[2] She married the glass painter Claes Pietersz van der Meulen, and their son Sieuwert van der Meulen also became a painter.[2] Her sister Heiltje married the painter Jan Slob, who had been a pupil of her brother.[3] [4] She died in Alkmaar.