This was a single-center, observational, prospective study designed to compare the effectiveness of a real-time, ultrasound- with landmark-guided technique for subclavian vein cannulation. Two groups of 74 consecutive patients each underwent subclavian vein catheterization. One group included patients from intensive care unit, studied by using an ultrasound-guided technique. The other group included patients from surgery or emergency units, studied by using a landmark technique. The primary outcome for comparison between techniques was the success rate of catheterization. Secondary outcomes were the number of attempts, cannulation failure, and mechanical complications. Although there was no difference in total success rate between ultrasound-guided and landmark groups (71 vs. 68, p = 0.464), the ultrasound-guided technique was more frequently successful at first attempt (64 vs. 30, p < 0.001) and required less attempts (1 to 2 vs. 1 to 6, p < 0.001) than landmark technique. Moreover, the ultrasound-guided technique was associated with less complications (2 vs. 13, p < 0.001), interruptions of mechanical ventilation (1 vs. 57, p < 0.001), and post-procedure chest X-ray (43 vs. 62, p = 0.001). In comparison with landmark-guided technique, the use of an ultrasound-guided technique for subclavian catheterization offers advantages in terms of reduced number of attempts and complications.