Introduction and Hypothesis <p>Perineal tears constitute significant complications of vaginal delivery, resulting in maternal morbidities. This study was aimed at determining the prevalence, risk factors, and morbidities associated with obstetric anal sphincter injuries (OASIs) in Southern Thailand. We hypothesized that specific maternal and obstetric factors increase the risk of OASIs.</p> Methods <p>In this retrospective study, data were collected from a database of pregnant women who underwent singleton vaginal deliveries between 2006 and 2022. The participants were divided into two groups: women with OASIs (cases) and those without OASIs (controls). The control group was randomly selected at a 1:4 case-to-control ratio. Chi-squared and <i>t</i> tests were conducted, followed by univariable and multivariable logistic regression analyses to identify independent risk factors associated with OASIs.</p> Results <p>Among 24,116 vaginal deliveries, 141 cases of OASIs were identified (0.58% prevalence) and compared with 564 controls. Multivariable analysis identified five significant independent risk factors: nulliparity (adjusted odds ratio [aOR] 4.47; 95% confidence interval [CI] 2.72–7.57 vs multiparity), forceps-assisted delivery (aOR 4.67; 95% CI 2.18–10.03 vs spontaneous vaginal delivery), prolonged second stage of labor (aOR 2.44; 95% CI 1.55–3.84), higher birth weight (aOR 1.70; 95% CI 1.07–2.72), and gestational age ≥ 37&#xa0;weeks (aOR 4.06; 95% CI 1.35–17.79). Documented morbidities included fecal incontinence (<i>n</i> = 2), stress urinary incontinence (<i>n</i> = 1), and rectovaginal fistula (<i>n</i> = 1).</p> Conclusions <p>Nulliparity, forceps-assisted delivery, a prolonged second stage of labor, higher birth weight, and gestational age ≥ 37&#xa0;weeks are significant independent risk factors for OASIs.</p>