William, it will not fit without an adapter plate, as the P365 series uses the RMSc footprint.
Adapter plate here budsgunshop.com/product_info.php/products_id/128972/trijicon+rmrcc+mounting+plate+sig+sauer+p365+xl+1-piece+matte+black ,
Stay safe & have a great day!

William, no this won't fit directly to the slide. The sig p365 series uses the Shield RMSc mounting footprint and this trijicon uses a rmrcc footprint. So you can mount this but would need an adapter. If interested here's a link to an adapter you can use to mount the trijicon rmrcc. Sorry but hope this helps, link here.................. budsgunshop.com/product_info.php/products_id/128972/trijicon+rmrcc+mounting+plate+sig+sauer+p365+xl+1-piece+matte+black

You will need an adapter plate to use the Trijicon dot due to it's foot print.
James G. Gave you more info.

Hi William and welcome. The Sig P365 uses Optic Cuts: Romeo Zero / Holosun 407k/507k / Shield RMSC. budsgunshop.com/product_info.php/products_id/137909/holosun+he507k-gr-x2+reflex+sight+multi+reticle - Cheers and stay safe! The one downside when it comes to mounting the Trijicon RMRcc on the Sig P365 is that you will need an adapter plate due to the unique footprint of the RMRcc. Adapter Plate for the RMRcc here : opticsplanet.com/trijicon-rmr-pistol-adapter-plate.html?sscid=81k7_vzx83&utm_source=shareasale&utm_medium=affiliate&utm_campaign=affiliate-1707605&sv1=affiliate&sv_campaign_id=1707605