Embodiments herein relate to an ostomy bag filter. In an embodiment, a filter assembly for venting gas from an ostomy bag is included having a first layer, an adhesive layer, an adsorbent element, and a second layer, the second layer configured to be welded to the ostomy bag at an annular weld area surrounding the second opening; wherein the adhesive layer is configured to adhere to the first layer, the adsorbent element, and the second layer, such that the first opening overlaps with the second opening; wherein the filter assembly is configured such that, when the filter assembly is welded over to the ostomy bag, gas from the ostomy bag flows axially through the adsorbent element and exits the filter assembly through the second opening; and wherein perimeters of the first layer, the adhesive layer, and the second layer are substantially aligned. Other embodiments are also included herein.