Usually no - a yeast infection is your own Candida overgrowing, not an STI, and you can get one without any sex at all.
Antibiotics, pregnancy, and diabetes are the common triggers. The Office on Women's Health names one exception: it is possible to get one from a sexual partner, though routine cases do not require partner treatment. The bigger issue is the look-alike problem - yeast symptoms overlap with STIs, so first-time or treatment-resistant symptoms deserve a real diagnosis.
What to do
Common questions
Is a yeast infection contagious?
Usually no, with one honest exception. A vaginal yeast infection is an overgrowth of Candida - a fungus already living quietly in and on your body - and the Office on Women's Health is direct: it is not considered an STI, because you can get one without having sex at all. Antibiotics, pregnancy, diabetes, and anything that shifts the vaginal environment are the common triggers, not partners. The exception OWH names: you can get a yeast infection from a sexual partner, and condoms and dental dams may help prevent passing it - so it is transmissible in the narrow sense, but it is not how most cases happen, and routine cases do not require treating a partner. What deserves more attention than the contagion question is the look-alike problem: yeast symptoms overlap heavily with other vaginal infections and STIs, and OWH warns that mistaking a more serious infection for yeast lets real problems run - so first-time symptoms, recurrences, or anything not responding to over-the-counter treatment deserves a proper diagnosis. Pymander, a free AI doctor available 24/7 by text, can help you tell the difference before you self-treat.