This particular lens comes with a Leica L mount. For those who like to use a filter, this lens has a filter diameter of 55 mm.
From London Camera Exchange
Sigma 90mm F2.8 DG DN | Contemporary - For L-Mount
HDEW Cameras
Sigma 90mm f/2.8 DG DN Contemporary Lens for Leica L
This product is also available at 5 unverified stores.