Background The purpose of this study was to investigate the incidence, location, and related factors of preoperative deep venous thrombosis (DVT) in patients with isolated patellar fractures. Methods Patients with an isolated patellar fracture, admitted between January 2013 and December 2019 at our institution, were retrospectively analyzed. Upon admission, patients underwent routine Doppler ultrasound scanning (DUS) of the bilateral lower extremities to detect DVT; those with DVT were assigned to the case group and those without DVT to the control group. Data on demographics, comorbidities, and laboratory test results upon admission were extracted. Differences between the two groups were evaluated using univariate analyses, and independent risk factors associated with DVT were identified by logistic regression analysis. Results During the study window, 827 patients were included, of whom 5.8% (48/827) were found to have preoperative DVT. Among those with DVT, 85.4% (41/48), 8.3 % (4/48), and 6.3% (3/48) occurred in the injured, non-injured, and bilateral lower extremities, respectively. Multivariate analysis showed that age (each increase of 1 year) (odds ratio, OR = 1.02), residential area (OR = 5.00), delay of injury to DUS (in each day, OR = 1.33), and elevated plasma D-dimer level (> 0.5 µg/mL, OR = 2.47) were independent risk factors associated with DVT. Conclusions Despite the low prevalence of DVT after an isolated patellar fracture, this study underscores the importance of identifying those with a high risk of DVT, especially those with multiple identifiable factors, as well as the early targeted use of thromboembolic agents, to reduce DVT occurrence.