The HTC One A9 seems to have been quite the flop. It is HTC's mid tier offering for late 2015. It looks eerily similar to the iPhone 6s, however HTC will swear to you that Apple stole design ques from them when designing the iPhone 6. That is neither here nor there. One great thing about the A9 is the fact that it was released with Android 6.0 Marshmallow.
The device has proven to be a bit of a pain to root thanks to the fact that it has dm-verity enabled and forces encryption by default. This means that you would ordinarily need to flash a custom system image prior to rooting. Rooting Marshmallow on the A9 is easier than ever now thanks to Chainfire's systemless root.
To root your device you will need to have an unlocked bootloader and TWRP installed. You will then flash the SuperSU 2.62-3 which is linked below. Before doing so you will want to be sure to fully nandroid backup your phone.
via XDA