<p>This study experimentally investigated the effect of coating thickness on phased array ultrasonic testing (PAUT) for defect detection in tower crane structures. To fabricate coated specimens with artificial defects, two types of artificial defects, surface notches and internal cracks, were machined, and each specimen was coated with a thickness ranging from 0.0 mm to 1.0 mm. PAUT analysis using A-scan waveforms and C-scan images showed that coatings between 0.1 mm and 0.2 mm provided optimal defect detection, with peak amplitudes of 25.1–29.2 dB for surface defects and 24.0–26.0 dB for internal defects. Beyond 0.2 mm, signal attenuation significantly reduced defect detectability. Thin coatings improved acoustic coupling, enhancing amplitude. Therefore, it was concluded that PAUT could be applied for the inspection of the tower crane structures with a coating thickness smaller than 0.2 mm.</p>