Catharina Oostfries, or Trijntje Sieuwerts (1636–1708) was a Dutch Golden Age glass painter.
According to Houbraken she kept up her drawing and glass painting into her seventies.
[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.