Hi,
I was reading the datasheet of the RB5009 and it says it supports 24V-57V in the DC jack. I was wondering if it's possible to power it with -48Vdc?

Best regards,

The most Mikrotiks have negative grounded chassis, so you can't simply swap the polarities. The only that has a true isolated input is the RB1100.

In this case good solution is to use isolated DC/DC converter.

If the device has negative ground, it needs to be completely isolated from common (positive in the case of so-called -48V systems) ground.

They are called -48v but there is not any actually negative voltage involved, as they were born for easyness/convenience in telecommunication installations an backup via a series of 6 or 12 or 24V batteries, it is just the polarity of the ground that is inverted, a low voltage insulated device, like many Mikrotik devices, in a full plastic enclosure, would have no issues with simply inverting the power poles, but the RB5009 has metallic case/parts that are inherently grounded (to the 0 or negative).

There used to be cars (mainly British I believe) that used positive ground, I learned the lesson a lot of years ago attempting to install a "normal", negative ground radio in one of those, the magic smoke came out in no time.