Mama June’s going after a dentist she claims promised to fix her teeth in exchange for some Instagram publicity, but allegedly pulled a bait-and-switch … leaving her mouth in major pain.

The reality star’s legal team has fired off a letter to a dentist in Beverly Hills. In the document, obtained by TMZ, June says she made a deal with the dentist to fly into town to get her chompers fixed … and all she’d have to do for payment is tag him and promote his work on social media.

June’s rep, Gina Rodriguez, tells us June arrived on Monday for her appointment and booked accommodations for 12 days per the dentist’s suggested timeline for the procedure to fix her top and bottom teeth.

According to the letter, the Doc removed her temporary dental implants from a previous procedure but then, “after placing her in a vulnerable (and painful) position,” told her he would not complete the process unless she agreed to do media appearances to endorse his services.

June’s team says this was never discussed at any point before the removal of her temporary implants, and now she’s scrambling to find a new dentist to complete the work because her mouth is exposed … and she’s in a great deal of pain.

June’s attorneys are demanding the dentist either complete the work as agreed upon — or pay her $35k to cover the costs of her travel and getting the work done by another dentist. If he does neither, they say their next move could be a lawsuit.

We’ve reached out to the dentist … no word back so far.