Jussie Smollett just took a big L in court ... because his conviction for his hoax attack was just upheld.

An Illinois appeals court handed down the ruling Friday, with the three-judge panel voting 2-1 to uphold Jussie's conviction ... according to new legal docs obtained by TMZ.

With the ruling, Jussie appears likely to return to a Chicago jail to complete his 150-day sentence.

Remember ... Jussie was initially convicted in 2022, spending 6 days in Cook County Jail before being released while trying to get his sentence overturned.

Now, Jussie's come up empty ... but his legal team has the option of appealing this decision to the Illinois Supreme Court, which would then decide on whether to hear the case.

During his appeal, Jussie dropped new music, directed a film with Vivica A. Fox, worked out with Taraji P. Henson, went to rehab and shaved his head.