This study focused on evaluating the diagnostic performance of the lung point (LP)- a lung ultrasound (LUS) sign in detecting and semi-quantifying pneumothorax (PTX), using computed tomography (CT) as the reference standard. The study included 150 patients who underwent CT-guided transthoracic biopsy (TTB) for lung lesions. Two radiologists blinded to the participant’s prior information performed LUS post-biopsy. The results showed that PTX was present on CT in 49/150 (32.3%) cases. The LP was positive in 36/150 (24%) patients, with a substantial agreement between the two radiologists (Cohen κ statistics = 0.8). The sensitivity and specificity of the LP were 73.5% (95%CI 66.5% to 80.5) and 100% (95%CI 97.6% to 100%), respectively. Moreover, the positive and negative predictive values were 100% (95%CI 97.6% to 100%) and 67.3% (95%CI 59.8% to 78.4%), respectively. In the semi-quantification of PTX, the location of LP was described in 36/49 (73.5%) patients. The sensitivity and specificity of this sign were 87.5% (95%CI 82.2% to 92.8%) and 96.4% (95%CI 92.4% to 98.9%), respectively. The positive and negative predictive values were 87.5% (95%CI 82.2% to 92.8%) and 96.4% (95%CI 92.4% to 98.9%), respectively. In conclusion, LP is a susceptible and specific LUS sign for diagnosing and semi-quantifying PTX.