Elon Musk jumped the gun on Russell Brand's sentencing ... calling the situation "unjust" before a judge even decided how much time the comedian would spend behind bars.

The billionaire weighed in on X after former British politician Andrew Bridgen shared a side-by-side comparison of Brand and Sajid Mahmood Khan, who received a three-year prison sentence for his role in a child sexual exploitation case.

Bridgen asked, "Are you going to see two tier justice in the Russell Brand sentencing?" ... and Musk fired back with just one word -- "Unjust."

But here's the thing ... Musk posted his response Sunday night, before Brand was sentenced Monday to 10 years and 6 months behind bars.

The comparison suggested Brand was facing harsher treatment than Khan, despite Khan's involvement in crimes against underage girls ... though Musk didn't elaborate on his response.

As we reported ... Brand was convicted Thursday of raping one woman and sexually assaulting three others following a four-week trial in London. He was acquitted of two additional rape charges and one count of attempted indecent assault.