A removable interior window trim system and method is comprised of either cut to size trim pieces or a pre-fabricated trim unit sized to insert within the jambs of a window and secure to the jamb and/or window frame and cover any existing mullions. The trim system can be secured in many ways, such as trim nails, adhesives, or spring loaded stops. The resulting trim system is particularly useful in creating a more expensive looking window during the course of window installation, but can also be used to retroactively improve the look of existing windows while allowing the user to change systems to create different looks without installing a new window.