Extreme hunger and poverty remain a significant barrier to maintaining a normal and healthy life, and increase the burden of tuberculosis (TB) in sub-Saharan African countries. The purpose of this study was to assess the prevalence and factors associated with undernutrition among adult patients with TB in Burkina Faso. In the Health Centre Region of Burkina Faso, we conducted a cross-sectional survey with face-to-face interviews of patients aged 15 years and older with TB (n = 302), from March to April 2019. Undernutrition was defined as a body mass index of <18.5 kg/m2. Multiple logistic regression analyses were performed to determine the correlates of undernutrition. The prevalence of undernutrition was 35.8%, in which severe, moderate, and mild thinness were 7.7%, 8.9%, and 19.5%, respectively. Low food frequency per day (adjusted odds ratio [aOR] = 3.28, confidence interval [CI]: 1.42–7.55, p = 0.005), human immunodeficiency virus infection (aOR = 6.18, CI: 2.26–16.88, p < 0.001), and diabetes mellitus (aOR = 17.52, 95% CI: 1.88–162.97, p = 0.012) and young age (45–54 years vs. 65 years and older, aOR = 2.93, 95% CI: 1.11–7.70, p = 0.029) were associated with increased odds of undernutrition. We concluded that the assessment of comorbidity and nutritional status should be part of the treatment for adult TB patients.